Well they are definitely bordering on unrealistic, but even they don't quite cross the line to "cartoonishly big". And it's more the shape than the size that gets unrealistic. Not impossible, though. Rare to an extreme, but not impossible.
But the question of how big is too big is very much a matter of persona taste and I can definitely see Sandy, Brandi, Nora and Bella being too much for some. However, even if that size were cartoonish, it's hardly fair to consider it a tendency when only a handful cross the line and 70% of the characters aren't even close to it.