Mercury was really into "bottoms", the fatter the better. Whether he was into female bottoms, we'll leave that for you to research.

A fat-bottomed person has a fat bottom. It's not a common expression but the meaning is clear. To be honest, I think the story behind this song is quite sordid and probably you can find out the details by searching online. The lyrics are not clear but contain innuendos. If you say that money or something else "makes the world go round", you mean that this thing is essential for the world to function normally every day. Its usage in these lyrics is an innuendo.

The great poet Sir Mix-A-Lot once said, "I like big bu*tts, and I cannot lie."