as you probably know, i have 2 kids.
my older son has ALWAYS been skinny. he looks like he's skin and bones. he was born early and it took him 9 months to go from the bottom 2% upto 50%, which is where he is now. we have a hard time buiying trousers for him because we need to have the adjustable waists. he's just tall and thin. he's 5.5 years old and weighs almost 40lbs.
my daughter, who is exactly 2 years younger, is solidly built. she's NOT fat, she's normal for her age, but because her brother is soooooo skinny, all of hte chinese people we know say she's fat. i HATE IT! she is average height for her age and weighs only 1 lb less than her brother.
i don't stress about it at all, it's just the way they were made...and they are so little yet, nothing is written in stone as to how they will develop over the next 10-20 years...