Hi KenH - welcome to the forum.
The oil brush currently simulates only a soft tapered brush, rather than a hard square-ended brush.
As a result, lighter pressure leaves a thinner stroke.
To make brushes bigger, if you have ArtRage 2.11, you can click on the percentage value in the stroke preview at the bottom left, and when the numeric entry pops up, type in a number up to 500% in size.
We put that in sort of as a test to see if people really wanted it. As it's proven to be so popular, we'll probably make the size buttons and the click-drag functionality also take it up to 500% maximum.
AndyRage's mantra for graphics engine code:
"Sure - how hard can it be?"