My son is almost 13 months and we've had a similar problem with him. I'm going to respectfully disagree with some of the other moms here.
It don't think it's always useful to force the little guys to sit in their highchairs because it turns meal times into a fight, which makes eating an unpleasant chore for them (I know, I tried it). I don't want eating to become a control thing for my son. Instead, I try to respect his independence by putting him in his highchair until he gives me the "all done" sign, then I take him out and sit him on my lap while I eat. He'll frequently take several more bites that way. Then I let him get down when he's ready and let him come back for more when he wants it. Or, if I feel that he really needs a little more, I offer him some more bites and bring him back to the table if he takes it. It sounds chaotic but, believe it or not, his eating habits have improved tremendously since I stopped the all-or-nothing approach.
I've also experimented with different kinds of finger foods that he can carry around with him if I think he needs to eat more over the course of the day: Cascadian Farms whole grain wheat cereal in his snack cup (he likes the crunch, and they're relatively low in sugar and sodium); lots and lots of fruit (I cut up peaches and melon because they're easy to pick up and go with); sandwiches. I know it's not ideal and it's better if he sits and eats but, like you, I want to make sure he gets enough and I'm willing to do what it takes to make that happen right now.
Also, you might want to avoid differentiating between snack and mealtimes. Some people prefer many small meals over the course of the day vs. 3 big meals. Maybe if you think about how much your daughter needs to eat over the course of the day and then prepare to offer her the option of a grain, a fruit, veg, dairy and even a protein at each meal (even those designated "snack" times) she'll ingest more in small amounts.