My 20.5 month old is pretty petite - she's just now hit 21 LBS and has always been around 10 - 15th percentile for weight. She's tall, though, and is probably in the 60th percentile for height.
I'm having such a hard time with sizing right now. 12 - 18 is getting short on her, but fits perfectly otherwise. 18 - 24 is a great length, but too big around the waist, arms, etc. ARGH.
So if you have or had a petite LO:
(1) How long and when did they wear 18 - 24?
(2) When did they start wearing 24 months/2T? The 2T stuff from baby Gap looks so huge - I can't imagine her wearing that when she's 2.