Getting a big and massive sounding theme or mix doesn’t need you to have a boatload of instruments playing altogether. What is needed is a strategic use of instruments that complement the other instruments that you already have in your arrangement.
Your goal as a music or sound creator is to consciously select instruments/sounds that you want to use in a given frequency gap. If you want to add power to a certain section of your mix, you should instinctively search instruments and sounds that have a naturally solid low-end response, thus getting you to your goal faster. Use the same strategy with sounds for the mid and high sections in the frequency range.
One effective way to train yourself is to listen to good mixes while you picture all the different instruments placed in the entire frequency spectrum.