Since it's an animated medium, it allows for more flexible physics, so boobs can bounce all over the place like
water balloons just like Mr. Fantastic from the Fantastic Four can stretch his limbs entire stories high.
In American comics, costumes are also very tight (sometimes, "skintight" is lightly put), which makes the
breasts/bottom appear even bigger/curvier.
Compare it to more "realistic" anime, where most girls have smaller breasts. Being truer to life also means
respecting real laws of physics and anatomy (most women have B or C). I wouldn't say the last image you linked to
is that bad, they're a bit larger than usual but nothing outrageous like in the first.
On the other hand, you also have the fascination with flat-chested loli characters (every harem anime has at least one),
so that would be interesting to look into as well. It's a matter of reaching extremes, though, of illustrating
something you don't normally see on the streets (in the case of bigger breasts).