Everyone is welcome to correct me if I'm off base, but you should possibly consider a male if you want to avoid a "fat" looking snake. They tend to keep a leaner profile than females. It goes without saying that you should still feed appropriately, of course.
If you want to avoid a "fat" looking snake then get a colubrid.
5.5 SNAKES
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2.1 Corn Snakes
1.2 Ball Pythons
0.1 Honduran Milk Snake
1.1 California Kingsnakes
1.0 Western Hog