Make paint go to edge of brush on photoshop CS4?

Whenever I use the paintbrush tool on photoshop, rather than using up the whole circle, it just paints with a smaller selection. Yea, I know I could just make the brush even bigger, but I’d still have to deal with this problem and it’s really annoying!
If you have any questions just post them and I’ll add them on with additional details. Please don’t forget to come back though.
I am not sure what you mean by pen pressure, and what I meant was the paint doesn’t fill the circle.