I have always read "a bit much" to be an idiomatic expression that really means "excessive" or "intolerable", especially with regards to behaviour. "His rudeness is a bit much." So would the opposite be "just right" (if there is an opposite?)"Not much" or "a bit less" are the opposites of "a bit much".
No, you are correct. "This watch is a bit expensive", "The meal was a bit greasy", and so forth. I was only mentioning the idiomatic use of the phrase "a bit much". Even then, I believe it's more British English than anything (not that there's anything wrong with that, of course!)I was thinking of the productive use of "a bit" + <adjective> to say that something is more <adjective> than expected/than it should be, e.g. "Your suitcase feels a bit light, are you sure you packed everything?" I think another way to say it would be "a tad much". But I guess with the adjective little it doesn't work and you have to rephrase.