Area brush bug

I was working on a large-scale map, so I wanted my trees to be size 10. I intended to create a large forest of pine trees, so tried to increase the brush area to 80. Instead, it increased to 7 area and wouldn't grow larger, even when I tried typing a bigger number in or moving the slider. I tried testing a different type of tree stamp, which gave the same results (only area of 3 instead of 7). Then I tried creating a swath of buildings at the same scale (size 10), and the area brush worked gave the same results (area of 26).