Are "No more healthy than" and "No more big than" both OK?

I am Japanese and a teacher of English. Now I am at a loss at a topic on "Comparison."

This sentence should be considered grammatically OK:

Oversleeping is no more healthy than overeating.

Whereas this sentence seems to be deemed wrong (including by AI-based grammar checkers):

This camera is no more big than my hands.

However, it does not make sense to me as these sentences should be identical in terms of their syntax and grammar.

Are both sentences correct?

Solution 1:

Quirk, Greenbaum, Leech and Svartvik have the following in their A Comprehensive Grammar of the English Language (pp. 462-463):

Most adjectives that are inflected for their comparison can also take the periphrastic forms with more and most. With more, they seem to do so more easily when they are predicative and are followed by a than-clause:

• John is more mad than Bob is.
• It would be difficult to find a man more brave than he is.
• He is more wealthy than I thought.

Periphrastic forms are, however, uncommon with a number of monosyllabic adjectives (including those listed in 7.75 as forming their comparison irregularly [good, bad, far]):

bad, big, black, clean, fair [colour], far, fast, good, great, hard, high, low, old, quick, small, thick, thin, tight, wide, young

They add the following note:

There seem to be fewer restrictions on using the periphrastic forms with adjectives in the comparative construction formed with the correlative the...the:

• The more old/older we are, the more wise/wiser we become.

BUT NOT: *a more old man

Good and bad, however, require nonperiphrastic forms (better, worse) even here.

The example you give fits the context in which fewer restrictions seem to apply to the use of the periphrastic form only partially. Big is used predicatively and followed by than, though not by a than-clause. Perhaps the addition of are could make it more acceptable:

• This camera is no more big than my hands are.

Solution 2:

Instead of using more to form comparatives, notice what happens when you use inflection for the comparative degree:

1. Oversleeping is no healthier than overeating.
2. The camera is no bigger than my hands.

More big is always wrong to form the comparative degree, which is always bigger .

More healthy could also be healthier but it does not necessarily need to be.

Solution 3:

I think the construction "X is no more A than Y" normally functions idiomatically to express "X is not A." You are saying some predicate does not apply in this case, just as it doesn't apply in this other case. The tacit part of the expression is "...Y is A." See below:

• "Oversleeping is no more healthy than overeating (is healthy)."

• "This camera is no more big than my hands (are big)."

It's not that the camera sentence is ungrammatical; it's just that it looks like it doesn't say what it intends to, which is that the camera is about hand-sized.

Here's a case where "no more big" would go through:

Child1: I'm big!
Parent: You're not big.
Child2: I'm big!
Parent: You're no more big than Child1.