With Apologies to Your Mind's Eye
I think the left's response to the charge of hypocrisy over smoker shaming would be that second-hand smoke harms the health of other people besides the smoker. I know there's some conservative pushback to the claim, but I've never conducted a survey of the necessary medical literature to come down on one side or the other.
Thus, unless Chris Christie walks up to strangers, pinning them down and force-feeding them in the same manner as a mother bird, the left would say that you're comparing apples and oranges. His diet doesn't affect the health of anyone else like second-hand smoke would, is the claim.
That said, there are certainly some on the left who agree with you entirely—that we should shame fat people alongside smokers.