Implicatures in Cartoons

Sometimes I THINK of keeping the cartoons I read because some are really intersting concerning 'implicatures'. I only read: Blondie, Andy Cap, Fred Basset and a few others -- Dilbert, too.

One from Blondie ran: Bumstead is ordering an order of hamburgher on the intercom outside a fast-food drive-in. "I'd like a big hamburger with a big order of french fries". Second square his wife stares at him. Third square: "But I'll order a salad instead."

------ The person taking the order -- you can see -- is at odds.

This is a case of an 'indirect' speech act. Surely he did not have to CANCEL anything, since, implicatures, unlike orders, ARE cancellable, by default.