All of the sentences in the OQ contain overt negatives, and in addition they're all comparative; and comparative constructions are Negative triggers, as is overt negation. So this is a very strong negative field, and strange things can happen.
In general, if there's a question about something grammatically odd, and negation keeps turning up naturally in the example sentences, you want to suspect the negation first, before ascribing anything to some other chunk.
In a negative field, quite so
X has a special sense, similar to very and other intensifiers. All of the following mean that he's not bright; the intensifiers are just there to intensify the irony (irony itself is a negative phenomenon).
- He's not so bright.
- He's not so very bright.
- He's not very bright.
Quite in a negative field means 'almost', in the sense that if we're not quite there, then we're almost there. It intensifies the smallness of the amount left. In a comparison like quite so much, the quite also emphasizes the smallness of the degree of difference. But it's not a grammatical phenomenon; it's semantic, due to the negation.