Negative versions of extreme adjectives

Solution 1:

I do not believe that a grammatical term exists for that. Terms like "synonym" and "antonym" exist because they describe relationships between words. On the other hand, the idea of something overly large or numerous describes an actual real-life condition. That falls within the province of vocabulary rather than grammar.

In other words, if someone told you "I am thinking of a word that is an antonym," you would have no way of guessing the word they were thinking of. However, if someone told you "I am thinking of a word that describes something undesirably plentiful," you could very reasonably guess "excessive." There is no need for a grammatical term for that relation; it is contained within the word itself, and not a relationship between words. The term "excessive" is the description for the condition it describes.

Solution 2:

There are words that deal with the varying degrees or scales of things but the generic term is simply "quantity". The most appropriate phrase for a quantity that starts causing problems is, simply, "too much".

Too much "security" is a "trap"

Too much "medicine" is "poison"

It isn't very romantic but it exactly matches your intended meaning. Aside from that, the word you are looking for would be anything else that describes a "greater" or "more powerful" form of a smaller thing. Mixing in the negative connotations and you get options such as:

• overabundance
• tipping point
• surfeit
• hedonistic

And so on.

Solution 3:

Juste milieu, the golden mean, is the the antonym of what you seek and as such something like ultra-milieu should confer the sense that it is an amount in excess.