pipe with smaller pipes, or install a demand booster pump, Tieger plumbing Installing smaller piping will NOT increase pressure it will increase the velocity (FPS) and thus can cause severe erosion of a ferrous and non ferrous metal piping. BUT decreasing the piping diameter you will lose a lot of volume. If you have 40 PSI incoming water pressure and increase the piping diameter to 2" you will still have 40 PSI and if you decrease the incoming piping to 1" diameter you still will have 40 PSI Installing a booster pump normally is not even to be considered in you had adequate pressure in the other fixtures in the home.
What you can do is 1- Check the shower head for restrictions such as 2- mineral deposits /flow restrictor 3- If you have a pressure / temperature protection type of shower body like a Moen or Monitor series of Delta shower body this can also cause a severe flow restriction. 4- Check to make sure all the valves are open leading to this shower body including the interregnal stops if applicable If you any more questions please feel free to contact this list Sylvan Tieger,LMP/ LMFSPC