Help me identify this spider!

This weekend I was visiting a friend in the country of NW Arkansas. On the top balcony was the biggest web spider I’d ever seen in person, almost as big as a cd.
He looked kind of like a crab without pinchers, and was spinning away at his web. His abdomen looked like a button mushroom, and his legs were fattish and yellowish. He was so strange!

Does anyone have an idea about what he could be?