Some rough sketches to accompany my story here:
http://z7.invisionfree.com/Too_Fat_.....?showtopic=360
If you're not a member, you can either join or you might need to wait awhile before I post a tidied, and maybe extended version here

In case you can't read my scrawly hand writing:
1: 'Normal' or 'Before' the vixens' normal rear-view. This accompanies the bottom-right picture.
2: 'Stuffed'- after eating a substantial portion of a large koi carp the vixens belly is bulging out to either side of her, blocking her egress from the chasm where she found the fish.
3: 'Stuffed AND Fat' she waited to digest the first portion of fish.. but was still hungry so snacked on even more! Not only does the stuffed belly push out to the sides (moving her legs with it and giving a pronounced waddle!) but it's encased in a layer of fat, which has also spread to her butt. This accompanies the picture at bottom left (roughly!).
4: 'Chubby' having let it digest properly and done a little excersise, she's still fat but not stuffed and hard. This gives a little pliability to, with the help of her long-suffering mate, squuueeeezzzeee her way back out of her predicament!