"And pigs might fly", though I think you have a direct Spanish one for this ... ? Or, from this, "Oh, look another flying pig!" ...? You might say "Of course, everything's bigger in Texas", though a Texan might (probably wouldn't) feel aggrieved. "Pull the other one" - a reference to "you're pulling my leg", has this don't lie/exaggerate + mockery element, perhaps ... "And I'm the Queen of Sheba" ... I quite like, from a pop song I think, "Sure, Pinocchio" ....