Quoting Javi's Mami*[Bazinga!]:" I felt like that too when SO went to the gym after work. He worked 7am till 5pm and then was at the gym ... [snip!] ... lol. So I think if he want's to work out he should be able to work out. Maybe he could get some equipment to have at home?"
I would definitely rather my SO gain a few pounds in exchange for more time together with our family.
I don't think your overreacting. Maybe you could compromise. A little less gym time. But of you try and take that away from him completely its could make him resent you or feel like you could be controlling him. I think it would be very fair to try and make a compromise though.