While I agree with you completely about the insensitive and utterly obtuse comment about your post-natal weight gain being hurtful, you must agree, in all honesty, that it was true. And it served as impetus to get back in to shape. I cannot comment on the motives of your friend for making such a comment, only you would be in a position to say whether it came from a genuine concern about your appearance, or whether it was a distinct but sly dig meant to be derogatory. Probably the latter from your post.
Wouldn't you agree that rather than objecting to people being judgemental it could be far more productive to come back with an appropriate and suitable reply that tells the person that they are being unnecessarily offensive and that its not OK. Even if its true, it could always be put forward in a way that is helpful rather than hurtful. That would be a positive way out of a situation which could otherwise leave you with a feeling of impotency and unresolved anger.
Believe me when I say, I am with you one hundred percent with you on this. Just offering a different perspective.