you don’t say so—is used to express surprise or sarcasm at what smb. has just said:
- I explained my presence, and he said: “You don’t say so! Why, I’ve got a patient over at Swampscott, who says he knows you.
you may well say so—what you say is perfectly correct, justified, etc.:
- “A craft that’s good for sore eyes.” “You may well say so,” replied the commodore. “There is no finer schooner-yacht afloat.”