Is the tank dimensions the only reason you can't keep an Oscar in a 55 gallon tank?

I've seen lots and lots of sights say that the minimum for them is 55 gallon but others say it's 75. Is the fact that the tank is only 12 inches wide the only reason you can't keep an Oscar in a 55 gallon tank provided that you have enough filtration and willingness to clean the tank out? This is a curiosity. I'm not going to try to stuff a full grown Oscar in a regular 55 gallon tank.