I also get annoyed when people start commenting on the weight of those they have seen a few times but don't even know. Body weight, for most people, is a very sensitive topic. Noticing is one thing, but I've never understood what makes people believe that it is appropriate to voice their opinions on the body image of perfect strangers. For that matter, I have PERSONALLY witnessed a situation where a man approached a lady at a grocery store and asked, "when are you due?" And, of course, her response, "Ummm, I'm not pregnant".
Sometimes people really are better off keeping their comments and opinions to themselves...