I once took a Psychology course which had another explanation for the large eyes. Apparently, when humans are sexually aroused, their pupils get larger. This makes people with larger pupils more sexually appealing, leading to all kinds of tricks to making your pupils larger (for example, candlelit dinners; also, in some older civilizations, women used to use eyedrops to make their pupils larger).

All of this leads to people thinking larger pupils look better, and in the case of small cartoon characters, look cuter.