It's offensive. The man has a problem, and is weird.
On the other hand, it's perfectly acceptable to ask a woman to lose some weight, if done tactfully and not making the woman feel guilty. Being thin is healthy. Someone asking you to make an effort to lose weight is someone asking you to be healthy.
If you encourage someone to eat better foods, and exercise, it's absolutely a good thing.
If you tell them they should watch more television and eat a couple bag of potato chips, a bucket of fried chicken and drink some soda, well, you get the idea. That's practically criminal behavior.