Why does my man expect me to be skinny when he is not?

Today my boyfriend told me not to use so much butter. I am 5'7" and 128 pounds. I feel like I'm an ok weight. On the other hand he eats fast food and oreos all the time and he has a beer belly. I have heard this happen to a few other girls too and I'm wondering...why does this type of man expect the woman to be in good physical shape when they are not??? He is a relatively nice guy usually... ?