For me, some days I'll eat a ton for my size (18 years old, 5'3" and 92 lbs.) and other days I'll eat next to nothing at all. I seem to be in that mode for the last two weeks now. I guess it's the change in weather or something causing me not to want to eat much. I'll be hungry, I'll take one bite, and I'm full the rest of the day. And right now I have the flu (grr...), so I don't even want to eat anything. And I think I may be losing weight. Plus I have an intensely high metabolism so whenever I do eat, it's really hard for me to gain weight in the first place. Yeah, I know, it sounds like I'm complaining over nothing...
I've always been really picky when it comes to eating certain foods. For instance, I hate chocolate, I've never been really big on sweets, I love meat (except for ham and liver), I don't like any vegetables (except dry spinach leaves in salad and green olives), I'll only eat a select amount of fruits, I'm not keen on cheese, I like hotdogs but not corndogs, I like hamburgers but not cheeseburgers, I like chicken strips with ranch sauce and not honey mustard, and so on and so forth. See what I mean?
Also, I've been pretty much raised on fastfood since my mom hates to cook. I've seriously eaten McDonald's nearly every day of my life since I was six! Another reason I probably haven't gained weight from all that exposure is because I limit my intake. Give me six Chicken McNuggets and a small water and I'll be fine.