Serena Williams -- My Ass Is Too Big ... To Run NYC Marathon

Serena Williams says there's a damn good reason she didn't run the NYC Marathon with her good friend Caroline Wozniaki ... "My ass is too big."

The tennis superstar talked her butt -- and a whole lot more -- during an "Ask Me Anything" thing on Twitter this morning ... even revealing that she "desperately" wants to have children.

Another interesting moment ... when asked if she was single, Serena wrote ... "Not by choice."

Sad, right?