Or ... perhaps women subconsciously select for preferred genetic material in their mates by using size as a visually measurable proxy for desirable reproductive traits.  Thus publishers simply mimic and reinforce a hard wired preference attributable to the species.

Just a theory.

Then again, maybe someone did a study and 4 out of 5 readers of such books picked the cover with the noted dimensional contrast.

Lotus 123 ~ S-Render ~ OS/2 WARP ~ IBM 8088 / 4.77 Mhz ~ Hercules Ultima graphics, Hitachi 10 MB HDD, 64K RAM, 12 in diagonal CRT Monitor (16 colors / 60 Hz refresh rate), 240 Watt PS, Dual 1.44 MB Floppies, 2 button mouse input device.  Beige horizontal case.  I don't display my unit.