May I use this kind of structure with "could(n't) have been" to express regret or satisfaction?
The cake was too small, it could have been bigger!
The party was great, couldn't have been better!
Learn English – Regret or satisfaction by “couldn’t have been better”
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Best Answer
Let's compare your examples:
This expresses an extreme. Since the party couldn't have been better, it's as good as it could possibly be. That makes this a fairly strong statement! It's even stronger than the party was great, so it works as a bit of superlative emphasis.
This doesn't express an extreme. The range of possibilities is anywhere from a little bigger to a lot bigger. As a result, this statement is fairly weak. It doesn't strengthen the statement made by the cake was too small at all. As it is, the second half of the sentence should be removed.
If you want to make a strong statement about the cake using this construction, you need to come up with a negative extreme that you want to express. (It doesn't have to be literally true; it can be hyperbole.) For example:
This is a little better, though it sounds a little silly. (It's not the world's finest example of hyperbole.) You could make a more general statement about its quality:
This is a bit less silly.
By the way, I'd like to recommend Daniel's answer, in particular for his description of "could have been better", a phrase which is intentionally weak.