OK -- I thought remembered coming across a lot of fawr seeming to mean 'not much' in the last Manon Steffan Ros I read -- Prism. Fortunately, I've got it as an e-book, so was able to search, with these results:
- There were a lot of examples that turned out to have explicit negative words elsewhere in the sentence (dim, heb, nad), such as:
Doedd gen i fawr o ddewis ('I didn't have much choice') or
Doedd o ddim wedi cymryd fawr o sylw o’r prism ('He hadn't taken much notice of the prism').
- There were at least a few where the only indication of any negative context in the rest of the sentence was a mixed mutation on the verb, such as:
chymerodd Math fawr o sylw ohono ('Math didn't take much notice of it')
Fyddwn i’n fawr o fam ’tawn i ddim yn poeni amdanoch chi ('I wouldn't be much of a mother if I didn't worry about you')
Os oedd hi’n dweud y gwir, fyddai fawr o ots ganddi petai Math a minna’n aros adref. ('If she was telling the truth, she wouldn't mind much if Math and I stayed at home.')
But I did find one good, clear example where there doesn't seem to be any negative word anywhere in the sentence, but the meaning does seem to be 'not much':
Roedd Math yn falch iawn o weld mai ond ambell berson a grwydrai’r strydoedd, a hynny’n dawel ac yn cymryd fawr o sylw o ddau fel ni. ('Math was very glad to see that there were only a few people wandering in the streets, and they were quiet and not taking much notice of two [lads] like us.')
My instinct is that it's like dim (or French rien) changing from meaning 'anything' to meaning 'nothing' from being used in negative sentences; but that the change is (as yet) only partial, and so has to be inferred from context. I'm also wondering vaguely, though, why it always seems to be fawr and not mawr...