A plump abdomen is a well fed spider but not necessarily a well hydrated one.
I almost lost a rose hair the other night because of an optical illusion. The water dish appeared to be full but it was actually bone dry! My spider was all clumsy and curled up so I opened the cage to see what the deal was... to my horror I realised that the dish was empty.
I quickly filled it with water and placed it right beside my spider and she almost immediately jumped in for a drink. By the next day the spider was back to completely normal and I learned a lesson (always look in the cage instead of through it because shadows can play tricks).
My point is that this spider was almost dead from dehydration... but the abdomen was large and plump.