The black singlet is the wrestling promoter's way of telling you that you need to lose weight god dammit! That's why Andre had to wear it in his later years, or the big show, or king Kong Bundy, etc
Fun fact, the guy behind Andre, Haku is possibly one of the toughest, most insane men to ever live. He could take any of the wrestlers on in a real fight, and his bar brawls would often end with someone's nose being bitten off, or the person getting tossed through a window. When cops would arrive, they'd find out that pepper spray and tasers didn't do shit to haku. There are some crazy stories about the guy out there