In my opinion if you don't feel they're fat enough @ the 65 day mark and you still see white pistils coming from the calyxes give them more time.
The 65 day mark is typically the earliest you can chop and get smoke-able bud. Autoflower breeders targetting the 'want it now' market will often list the earliest possible date to harvest as opposed to the most optimal date to harvest. I had a 60 day wonder I let run for an additional 3 weeks and the buds fattened up so nicely and the smoke was absolutely delicious.
You'd be amazed what these plants will do as a 'survival' tactic at the end of their life span, they'll use every nute and resource available to them!
Of course if you have a microscope, that's the absolute best way to go =P