Here is the final, most likely explanation for the cuteness paradox:

1 - Cuteness genes are positively selected by many things, but the main filter, at least in mammals, is THE MOTHER INVESTMENT. Puppies (humans, bunnies, all of them..) compete for the investment of the mother, because she is the one that feeds them. They cannot feed themselves until they are adults. Cuteness is a deceivement device and (because it costs physical resources) an honest signal for communicating the mother that the deceiver is the puppy most worth of the maternal investment. Even non mammals use cuteness (i.e. birds and other oviparous species) but their cuteness is rarely perceived as so because of the huge physical differences with the mammals, differences that our mammalian brain cannot see as cute but as deformities.

2 - The selective pressure is bigger when the number of puppies is greater, because the competition is more tight.

3 - Female bunnies bears more children. A litter of rabbit kits (baby rabbits) can be as small as a single kit, ranging up to 12 or 13; however there have been litters as big as 18. So the competition is harsh, and consequently the selective pressure on the cuteness genes is bigger.

4 - Women give birth to 1 or 2 children at once on average, consequently the competition and the selective pressure on cuteness genes is greatly inferior to the selective pressure on the bunnies.

5 - Assuming that cuteness is an universal estetic trait (big facial elements, head bigger than the body, small arms, etc.), developed at the same way in all mammalian brains, it is then reasonable to conclude that human babies display cuteness traits, but are not as cute as the bunnies, because those are subject to a much more tight competition.

Findings that can falsify this hypothesis: - the existence of a species of mammals that bears many children at once that are not cuter than those that bears few, provided that those are normally competing for a shortage of resources from the mother. We need to take in consideration other factors as well, like shortage severity, likeness of the mother to drop some of his puppies if attacked or in extreme hostile environments, and so on.