How Does an HOA set their fees?

This question is submitted by Griffin J. out of St. Louis, MO 

I live ins a strange HOA situation. Most of the units are between 1,000 - 1,500 SQ.FT. there are a few that are larger ~3,000+. The issue that I find is that we are being billed differently for our monthly dues. I pay $250 a month whereas the larger units are paying $350 a month. Is that normal that the larger the space the more they pay? is that legal