Overweight Burmese Python?

[Will's Reptiles]

Yesterday I got out my baby burmese python after feeding him on Monday and he seemed a little chunky at the base of his tail/around his vent. I had noticed it a little bit when I first got him but yesterday it seamed way bigger. He is being fed one small adult mouse every 7 days and gets out every day except for the day after he gets fed so I don't know why he would be overweight. I can still slightly feel his spine when I run my find down his back and he seems pretty muscular except for right at the base of his tail. Let me know if you have any ideas what it could be and it is just fat deposits, how do you get rid of them?
Thanks!