If you're using the same brand of powder and you change nothing else--same hull, same primer, same wad, same shot weight--but only increase the powder charge, you're correct. More powder = more velocity and more pressure, assuming nothing else changes. However, you can get more velocity with less pressure, or less velocity with more pressure, if everything remains the same BUT you switch brands of powder even if you keep the powder charge weight the same. That's because of faster vs slower burning powders.