How often are you feeding her throughout the day? Does she snack on things like crackers? How often/how many? Do you put just a little food on her plate at one time, or alot? Does she only sit in her highchair for meals?

One thing to remember is that the amount of food a toddler needs to eat to be full and thrive is much, much less than you or I would imagine! She won't starve herself, but I understand your desire to have mealtimes be less frustrating.