When I was testing around with wraiths in creative I managed to spawn a few with a hitbox way too small. It seems to be on a low chance but is this maybe some kind of baby-mob variation stuff left over?

To reproduce:
Boot up game with quark 159 and spawn in a lot of wraiths, every now and then you get one with a hitbox that is way too tiny.

Is this because you right clicked another wraith with the spawn egg?

Is this because you right clicked another wraith with the spawn egg?

No, I would have mentioned that.
It's purely by chance which is why I was thinking about the baby versions of hostile mobs rather than the mechanic you mention which seems to be only for mobs you can breed.
Just tested, baby zombies work the same way, you cannot spawn them by rightclicking a normal zombie with spawn egg.

interesting...