Actually, you would go up to a larger diameter prop when going down in pitch. There is a certain amount of prop that your engine can turn. You can use that resistance in either pitch or diameter. A bigger prop grabs more air, therefore it pulls harder. But to keep it from overloading your engine, you have to go down to a lower pitch. A higher pitch prop moves more air, so it can get the plane going faster at the expense of all out pulling power. Going with a low pitch and small diameter (underpropping) lets the engine get its RPM up quickly and operate at a faster RPM. That comes in handy when you're doing 3D.

For a typical .40 size engine, common props would be 10x7, 10x6, 11x6, 11x5,  and 12x4. From left to right, you're going from top end speed props to acceleration and vertical performance props.