Nothing about my comment assumes that the "size of the pie" can't grow. It can, and it does. But if the wealthy take all of the increase in the pie for themselves (plus more!), then the poor and middle class still get screwed over, and that is exactly what is happening in the U.S.
For particularly shameful evidence that wealth in the U.S. is not being adequately spread around, see this article: https://www.theguardian.com/us-news/2017/sep/05/hookworm-low...