I don't know if this is a bug, but when I was trying my mod I accidentally pressed F3+B, the button for showing hitboxes. I was fighting against a mob with the Silverfish model, when I noticed that its hitbox was as big as a Zombie hitbox. In fact, I could hit the Silverfish-like mob in the air because its hitbox was a lot bigger than the mob itself. I tried to spawn a vanilla Silverfish, and I noticed that its hitbox was fine.
The same happens with chicken model mobs. Their hitboxes are too big.
There is a similar bug with slimes that I found a lot of time ago but I always forgot to report it.
When you make a mob with a slime model, it always spawn as a little sized slime (that isn't a bug) but its hitbox can be big, small or medium. I think that it's only that the hitbox changes based on the slime size, but the model doesn't.
(Sorry for bad English)
Edit: fixed some grammar errors