This is because pumpkin puree generally makes a cake-like texture when baked and brown sugar also causes cookies to be less thin, so the combination of the two often leads to really thick, cakey cookies. Using white sugar increases the spread of the cookies, because the white sugar dissolves while baking faster, helping compensate for the puff you see when baking with pumpkin puree. So the cookies puff less in total. It's baking science, baby.