I'm mad that my fiance lied to me about not losing weight, so I'm kicking her out and planning on breaking up with her
Why do you want to kick her out?
I don't like fat people and she's not losing the weight.
It is not right to lie to a significant other about weight loss. But in this situation, maybe you can consider giving her a second chance. You are already engaged and maybe you guys can come to an agreement.
She will just blow up again and gain the weight back if I don't do this. I just want to burn her house down instead of giving her a second chance.
If you really feel like your fiance violated your trust, then it's OK to end the relationship with her. Just keep in mind that sometimes it can be hard to lose weight, and she may not have lied to you on purpose.