There is zero extra room anywhere in the F-16. It's like the fighter planes of WW2. You don't sit in it as much as you wear it around you. It's surprising to walk up to it and see just how compact it's all put together.
I sat in an F-16 and have flown the simulator. The seat is high to give the pilot a better view and the seating is tight, not just in height but also shoulder width. We didn't see anyone with any excess body weight in the fighter squadrons despite most of them being Guard and Reserves.