I know that a football ball isn't a perfect sphere, but if you look at photos of balls in a similar resolution of the icons, you'll see that the distortion is so low (if even perceptible) that it will look almost like a circle (examples:
here,
here and
here).
It's also the reason that in-game (where the resolution rendered is even slower than the posted original ones, if I not mistaken), the deflated effect will seem much more attenuated (if even so perceptible at all, as jymis said).
Anyway, as I've said originally, the new ones are much better than the old ones, even if deflated
Edit: With the help of Gimp's ball pump, I tried to quick inflate them (made than real circles). Not sure if better...