Move Or Enlarge Hitbox Of A Mob?

If any of you know, you may or may not, i have added into my Anime World mod colossal type mobs. Now the problem is, their hitbox is right at their feet and i want to move it up closer to their head (specifically their core, but that's right below the head). Does any of you know how i could do this?

Here are some pictures to explain what i want to do:

as you can see, the hitbox for sachiel is small. Too small. What i want it to do, is to either take up the entire size of sachiel or just the core (which isn't in the picture because it couldn't fit. Shows just how big Sachiel is)

Another example is here with the Cherubim Soldier which is coming in the next update of mu Anime World Mod:

The same thing applies here, the hitbox of the mob is too small compared to the actual mob. (and the rest of Sachiel is in the background showing the core which i initially want the hitbox to be at)

If anyone knows a way to move the hitbox or make it bigger, please tell me!