For the most part, it's the federal requirements that restrict the max flow from a showerhead along with vanity faucets and low-flow toilets. Some of it is design, some is the available water pressure, and some of it could be the type of pipe used (cpvc and pex can't flow as much as copper, for example). Do you know what your water pressure actually is? Do you run both of them at the same time? Is either one or the other okay, but not when it's two together?
Some of the designs have an easily removed flow restrictor, on some, if you're willing, you could drill out the inlet opening. But, for the most part, you may need to live with it. Fair warning, removing the restrictor will drain your WH quite fast!
If your incoming water pressure is unusually low, and the supply is not restricted (just low utility pressure), you could add on a booster pump for the house...sort of like a small well system, but it's self-contained. This Olde House has one they did in their archives you could watch online to see what I'm talking about. Pretty simple to add. While many people live with incoming water pressure in the 20-40# range, 60# or so is nicer for things like showering.