Here is a gringo on Copacabana beach. Note the extensive fabric in her swimsuit bottoms. This is known as "full coverage". This is a faux pas in Rio. Locals wear something entirely different here, to cover their entirely different anatomy.
You may have heard of the Brazilian bikini. Here are a few pictures to illustrate the style. They are uniquely designed to highlight the gluteal area. Because it is the fashion here, almost all the women on the beach wear them.
Most of the Brazilian women who wear these look great in them, butt not all.