Not so much fat as a giant in normal clothing. As another reviewer puts it, the sizing is waaaaaaayyyyyyyyyy off. I wear a medium in everything and I could barely even zip it up without the bladder in. Once i finally put the bladder in and delicately zipped up the shirt as I was about to go out the door for a ride to try it, I decided against. Not only did I look like a Meathead in sausage casing, but I think it would have choked me after 30 minutes. Unless you are a rail thin cyclist with no muscle mass what-so-ever, size up, and despite that it still felt like I had a sloshing hump on my back. There is only so much that shirt fabric that is designed to breath can do to keep a water bladder in place I guess. I was hoping for a great product that I could take on the trails with my bike and maybe even run with, but it sloshed around too much and ended up just going back to my normal camelbak trail pack. Had to return it and wouldn't recommend.
Sincerely a man that is 5'9 that weighs more than 155lbs and who doesn't have a chest like a middle school girl.