D.P.
Kids have this useful little mechanism that causes them to stop eating when they are full. (Don't I wish I did!) She's fine. They can also eat like snakes--hardly anything O. day, then super consumers the next.
She also might be a grazer. Offer frequent healthy snack, especially if she just about "skips" a meal with such a small portion.
All kids are different and it can be scary moving from a "set" amount of formula etc. where you can easily measure what you think is "enough" to a more varied diet.
Best thing you can do is make sure that what she DOES eat counts well nutritionally.