T.H.
Well, hmmm, the thing is most toddlers don't have the same issues with food as adults so they will usually cut themselves off when they are full. They do generally eat quite a few times a day and they usually only ask for food if they are really hungry. Now, I will say that even my 2 year old will say he's "hungry" if he spots some cookies or other sweets that he'd like, but I've learned to read those cues pretty fast! ;)
What is she eating that she wants more of? If it's healthy food like proteins, veggies, fruits, etc. I'd honestly just let her have it. Maybe she really is hungry. I honestly believe that at 21 months, she is not trying to manipulate the situation or over eat. I would give her more food as long as it's "good" food and I bet she'll stop when she's full.
Also, does she drink a lot of milk? If not, then maybe she needs to get full off of other foods. At that age my kids were still drinking whole milk so that curbed their appetite a bit, but once that stopped they ate more foods. So if she isn't drinking a lot of milk, or drinking a lower percent, then again, probably hungry.