For health stuff, I think if you're between sizes you should order a bigger one.
I had a few different Underworks binders and the sizing stayed consistent for me. I did get a bigger size though, because I was between a large and a medium. The only measurements that changed significantly for me were my shoulders. With starting T and getting more consistent at the gym, my shoulder span increased by several inches. The large still fit, but was much tighter around my shoulders than before.