If you bought your cage at a pet store, with very very few exceptions (they may exist, but I've never seen one), you can be very sure it isn't big enough. 7.5 square feet is minimum for a single cavy, 10.5 is what is preferred for 2 (and bigger yet is even better). And unlike rats or other small animals, what matters is the ground level. Additional levels are great, and if they use them (one of my two boars does, the other doesn't), they are useful for allowing two cohabitating cavies to get away from one another and add more interest to their environment, but they don't count toward the minimum space requirements.
If you build a new cage from the storage cubes, though, what you can do is use your current cage AS the second level, or as a quarantine cage for any new buddies you get for your current piggy.