Big Man Syndrome (BMS) is when you are a bigger person and you feel the need to overpower a situation just because you are a big person. It can happen in someone that is fat or someone that is muscular, most commonly found in muscular people.
Someone hits the pingpong ball over the net really high and you know you can easily score but you hit it as hard as you can just because you have Big Man Syndrome.
Big Man Syndrome (BMS) is marked by delusions that because there's a girl willing to sleep with you and hang on your every word (hence being a "big man") that everyone else, particularly males, are equally enthusiastic supporters of the mindless bullshit you purport. Other symptoms include: unwillingness to listen or compromise, unreasonable demands you expect to be fulfilled and the belief everyone else shares your interests.
"Man, Sean won't shut the fuck up. Ever since he started dating what's-her-face he's had some serious big man syndrome."