I love the little bit of Fridge Brilliance embedded in that line. Why would the large women get the most snu snu? Because they made the decision - the large women are probably the ones in charge of the social pecking order. And it makes perfect sense that the largest women of Amazonia are in charge. Their physical strength means they can literally push others around, and in a society of humanoids that’s become as giant as they are, it’s likely that the largest women have been prioritized for mating rights as well.

In this same light, it makes sense that Zapp, the largest of the three, is snu snu’d by the large women, while skinny Fry gets the petite women. The large women have the best chances of having large offspring by snu snuing Zapp. The petite women probably have less say than the rest, so they’re given the scrawny guy. Everybody wants Kif, but the large women still demand more snu snu than the rest, so they get him twice.

Edit to add: a direct link to the Futurama page on Fridge Brilliance, simply because it’s much more fun than doomscrolling and I figured others might enjoy it, too. The thing I said isn’t on there, but the page does offer this:

In “Amazon Women in the Mood”, the ruler of an all-female society describes Kif as being the most attractive of all the men. At first, it just seemed like a humorous joke, as Kif is a goofy-looking alien, and not exactly the epitome of masculinity. But this panel from the comics gives a decent explanation as to why the women of Amazonia were so charmed by Kif. Being effeminate and girlish are traits that are typically seen as undesirable in men on Earth but would definitely be desirable traits in a society populated entirely by women.