I have a friend who does this all the time.
She is tiny, in a gorgeous way, and cares very much about staying that way (think Posh Spice kind of woman). She can bang on about portion size, when we have fish and chips she is "shocked" at the portions, and always says her family eat a portion for 1 between the 4 of them.
She will eat half a pita bread with some salad for lunch. or at a BBQ, half a sausage and some salad.
Funnily, her DH is very skinny too, always ravenous, and goes to the 7/11 for secret top ups and snacks.
She just hates being fat, and is very determined. It makes me aware how much work it is to be a size 4/6 aged 40, and makes me actually feel I get the better deal (eating nice food).
I don't mind, it's just how she is. But she is now putting her normal (completely normal, slim but not skinny as her mum) sized 11 year old DD on a diet, and we (friends) have been challenging her on that!!!