Cut lines getting wider when cutting

I am trying to cut out a design and it seems like the laser is making the cutout lines a little bit wider. I am doing 3/8 wood with power at 40% and 500mm speed. Attached is two images to help illustrate. The rectangle on left is original 2 passes and the one on the right is pass three, four etc. As you can see the line is wider. Any idea what may cause this? thx