Eye candy.
Correct me please if I am wrong, but I always thought (and I thought I read) the opposite.
A fish appears the biggest in a spherical fishbowl, which acts like a convex lens and gives a magnified virtual image of the fish because of the refraction.
In a perfectly rectangular tank, the fish only appears at a smaller depth than they really are, that is closer to you and so does the back wall as well. But the glass panel bends and this places it between a bowl and a perfectly flat rectangle viewing panel. Hence, the more it bends, the larger the exaggeration.
http://scienceline.ucsb.edu/getkey.php?key=1783