My pet wither skeleton is huge!

I used Tynker to modify the wither skeletons to be tamable and act like zombies. i have lots of pet wither skeletons since I added the behavior pack. Most of them are about the size of a regular zombie, but i have one that is especially HUGE in comparison to the others. I copied the link:

https://www.tynker.com/get/x608n7ro 

the add-on is called "bug-eyed wither skeleton". it is somewhere in the community page. search "bug-eyed wither skeleton" and you should be able to find it. I also downloaded a picture of the huge wither skeleton from my portfolio. 

here is the size of the others in comparison:

I also have a baby wither skeleton. here is the only one I have:

I'm really wondering about the huge one. It's the biggest one I have. Like i said, i modified it to behave like a zombie. So why is one of them so huge!?