How can something be "fuller" or the "fullest"?

Solution 1:

Considering that "full" is often used linked with a description as to the degree of fullness, it seems reasonable that there can comparisons between these. For example, something that is 90% full is definitely fuller than something that is half full.

Solution 2:

TL;DR The adjective full is not a purely yes–no characteristic in all its senses, and fuller and fullest are perfectly grammatical. Even if it meant only that, they would still be grammatical.

Your error lies in thinking that full can only mean “at the maximum”. Amongst many other things, it can also mean how filled or how complete something is, which is clearly gradable by virtue of the “how much” aspect.

For example, here is an abbreviated selection of the OED’s many senses for full:

1a. Having within its limits all it will hold; having no space empty; replete.

2a. Containing abundance of; plentifully charged, crowded.

2c. In non-material sense: Abounding (in), abundantly characterized (by). Const. of, occas. †with

4a. Having eaten or drunk to repletion.

6a. Abundant, amply sufficient, copious, satisfying, satisfactory. Said both of material and immaterial things.

6b. Of an account or report, hence of a writer, etc.: Complete or abundant in detail.

7a. Complete, entire, perfect.

7b. Answering in every respect to a description; possessed of all the qualifications, or entitled to all the privileges implied in a designation.

8a. Complete in number, quantity, magnitude or extent; reaching the specified or usual limit. Of the moon: Having the disc completely illuminated; cf. full moon. Of the face, or front: Entirely visible to the spectator;

9a. Possessed of, delivered with, or exerting the utmost force.

9b. Of light: Intense. Of colour: Deep, intense.

10a. Having a rounded outline; large, swelling, plump, protuberant.

10b. Of portions of dress: Containing a superfluity of material which is arranged in gathers or folds.

Notice how many of those make complete sense when gradable. Provided citations include:

• The longer a ship is, the fuller should be her Bow.
• In the spring a fuller crimson comes upon the robin’s breast.
• He proved the fullest rogue··in either kingdom.
• The fuller it is of labour & slavery.
• You will find in its columns all the latest and fullest telegrams from every part of the world.
• The audience are quite at liberty to take the fullest notes they like for their own personal convenience.
• The case is reported··by Lord Raymond, whose report is the fullest.

There are therefore endless ways that full can reasonably inflect into fuller and fullest. Certainly the senses related to “plump” or “abundant” or “plentiful” or “ample” all work for this.

Here are just a few, all perfectly idiomatic:

• Women of fuller figures go to different shops than those of slimmer figures do.

• The moon can be fuller tonight than yesterday, but it won’t be at its fullest point for a few days yet.

• Certain preparations can make your hair feel fuller, just as certain foods can make your stomach feel fuller faster and longer than others can.

• Deep study can lead to a fuller understanding of the problem.

• You can live life to the fullest.

• Perpetrators will be prosecuted to the fullest extent of the law.

• You could have a contest for who has the fullest beard.

Even if full really did mean only 100% complete and never anything else, it is a myth than adjectives that appear work like toggle switches, either enabling or disabling some property of the modified substantive, can never admit modifiers of degree.

• In order to form a more perfect Union, establish Justice, insure domestic Tranquility, . . .
• He was quite dead, deader than a doornail.
• My wife is so pregnant we keep the hot water running all the time now.

Solution 3:

There are at least two sense in which "full" is not a superlative. It can mean crowded:

The street was full of people.

It can mean rounded or swollen as in "full-figured."

The comparative and superlative forms will work for those.

Solution 4:

While the concept of "full" seems pretty simple, in practice it is not. A humorous illustration is the old joke that goes:

A philosophy professor set an empty jar on his desk in front of the class.

He proceeded to take a few large rocks and put them in the jar until they reached the top.

He asked the class, "Is this jar full?" The class all agreed, the jar was full.

Then he poured in some small stones, shaking the jar as he did. Again, he kept at it until they reached the top of the jar.

He asked the class again, "Is this jar full?" The class all agreed, the jar was full.

Then he took some sand and carefully poured it in and shook it until the sand had filled in all the spaces between the rocks and the stones.

He asked the class a final time, "Is this jar full?" The class all agreed, the jar was full.

He then took another jar, and filled it with sand. He then asked the class if it was possible to add some large rocks, or even some small ones. The class agreed, the jar was full and no rocks could be added.

He explained, "These jars are like your life. The large rocks are the most important things in your life, like your job and your family. You can fill up your life with just those. The small stones are like your dear friends; they, too, can fill up your life. The sand is like your hobbies and other interests; they will also fill up your life. But note that you can get the order wrong. If you fill your life with the little things there will not be any room for the big ones."

He asked, "So, class, what lesson did you learn?" One student raised his hand, and the professor called on him. The student came to the front of the class, pulled a beer out of his pocket, and poured it into the first jar. Then he explained, "However packed and full your life may seem, with big things and small, there's always room for beer."

Solution 5:

The word full is often used even when the limit of the capacity being described is unknown or unknowable, or even if it simply doesn't exist.

For example, "She lived a full life, but her mother's life was fuller" is perfectly acceptable: how "full" can a life be? Even if you figure out how to break it down to the number of things a person could do per year, fullness of life is more than just doing a lot of stuff: someone who hardly does anything might be said to have a more fulfilling life than someone who's constantly busy.

Or consider the way liars are sometimes described as full of "it" (where "it" might be any of a number of ridiculous things). One person could easily be described as "even more full of it" than another if they are more deceptive or pretentious. Just how deceptive can a person be, anyway? Sure, the most deceptive person literally would never tell the truth, even to themselves, but that person is clearly and simply insane to a degree beyond the descriptive power or sensitive use of "full of it", and the line between "insane" and "ridiculously deceptive" is not fine or clear.

In short, full isn't necessarily, or even usually, used in the most absolute sense. Only the Sith deal in absolutes.