i'm not entirely sure, but a few suggestions:France has a much larger military than I do (49k vs. my 30k) but they are not insulting me or anything (Relation a +79 and rising). I'm happy about this but I never expected nor truly wanted the PU to hold since it would remove all challenge and frankly France has still not absorbed it vassals so it would be a bigger hassle overall.
Anyway am I missing something? What determine if a country stays or not in a PU?
Your'e not at war, are you? Lesser partners don't send insults for as long as you're both at war with someone else.
I suspect the AI looks at total military strength not just army size. Do you have a huge navy? Has France got no manpower left?
It might just be that they are using their diplomats for something else (raising relations with their vassals, maybe) and haven't got around to insulting you yet. Try saving, quitting the game (all the way to Windows) and reloading, that might reset the AI.