Besides what Steve mentions, there is a somewhat more involved approach you can take. In the table FCT_ShipDesignComponents you can edit the ComponentValue column values for each type of Cargo Hold. You can use the relationship that 1 unit of minerals = 2 cargo points to revise the ComponentValue so that each cargo hold can contain the number of minerals you think is reasonable for your desired game type.
After doing this, you almost certainly will need to follow Steve's advice and modify all of the installation sizes in DIM_PlanetaryInstallations. If you do not do this, it is almost certain that you will break the NPRs as they will not be able to ship installations to outlying colonies effectively. As always, modding the DB means you cannot submit bugs from a game played with that DB unless you can verify them in an unmodded version.