Does it really bother you that much? From IS' post, it seems like a creative spark from a -female- (note she). And it technically is a new body type. Half fish, half human. Really, unless you're a female who has huge problems with society, this shouldn't be an issue.
If we have too many female child body types, then we'll have people calling Riot out for being pedophiles.
If Riot makes too many flat chested girls, then they're lolicons.
Get the point? If Nami isn't satisfactory to you, then I don't know what to say. Sure, her boobs can be smaller, but it's to make it prominent that she's half female human and half fish. Unless you can think of a better way to show that she's a female besides voice/face structure (and even then it's "generic" - thin face)... or you can make her a fat whale. Not a lot of variety tbh.
Honestly, if anything, Riot probably has to make all male champions from now on to keep people from making nonsense posts about this.
These "feminists", and I put that in quotes because they're not really feminists at all, more like anti-*** fundamentalists, are incredibly dishonest and basically just get pissed off at any depiction of a female character. Riot please ignore these bad people and let your artists design characters properly with no input from rabid morons.