Ok, the real answer is I draw Billa with childlike proportions because I find that cute, whereas Luri looks like a teen or young adult. So I'm going to be shorter. The mouse vs pony aspect is still there if you still think the size difference is too much, though in the furryverse that doesn't really matter.
Though, sometimes that size difference is used, like in Zootopia http://i.imgur.com/vYLy41K.jpg
I think the only size thing wrong here is how clunky big I made Luri's hooves. I was conflicted about whether to use hands or hooves, and ended up just drawing hoof shapes over the hands I had drawn at the last minute.