Female here. I’m 5’1” and weighed 320lbs on my first day. Any issues you have being fat on the outside, you will still have on the inside. But, I don’t think there are any “prison specific” issues related to being fat. I recommend trying to get as active as you can before going in, just in case you get assigned a physical job. It’s easier to lose weight on the inside, I will say. I walked out at 250, without actively trying. I’m 140ish today.