A voice has a set of parameters-- e.g., volume, octave, pan, reverb level, etc. Each preset voice has certain default values for these parameters, and selecting a new preset voice will change these parameters to the default values used with the new preset. As you've discovered, the default volumes for the preset voices tends to be all over the place-- some voices are loud and strong, while others are quiet and soft.
Fortunately, after you select a preset voice you can adjust its parameters to whatever values you prefer. Note that each keyboard part-- i.e., the Main Voice, Dual Voice, and Split Voice-- has its own set of parameters, so when you select a given preset voice for a given part you'll want to adjust the parameters for that part. See pages 72 - 75 of the Owner's Manual for a list of parameters you can change via the Function menu, and page 60 for an overview of the voice settings.
Obviously, you don't want to have to change a bunch of parameters each time you want to switch to a different voice while playing. What you should do is set up everything the way you want, then save your settings to a registration so you can recall them when desired; see pages 50 - 51 for more information. Set up the first voice (including all of its parameters) that you'd like to use while playing a particular song, and save it to a registration. Then set up the next voice and save it to the next registration; etc. That way you can recall each registration setup while playing and the voice volumes will be set to your preferences.