It simply asks the question, "Did you actually do that?", however, it drips with sarcastic disbelief or belittlement.
Alternates are "Do ya?" or "Will ya?"
Henry: "I've given up drinking."
Jimmy: "Did ya? I'll buy you a beer at Mulligan's and you can tell me all about it."
Horace: "I have an IQ of 133."
Jimmy: "Do ya? Then why is your shirt on backwards, stupid. Ha, you looked."
Headley: "When I die, I'm going to have my remains cremated and compressed into a diamond!"
Jimmy: "Will ya? It looks like the process has already started on your head, precious."