My understanding: A "Universal" app is really two bundles of code and it's associated resources ( which may be usable by either code set ). Obviously, intel code will not run on a PPC chip and vice versa, thus the need for two code bundles. Since many of the 'resources' are just text or bits in a particular order, either PPC or intel code should be able to use them. Therefore, a Universal app will be larger than either a pure PPC app or a pure Intel app. But you don't need to worry about what's what as the OS on the machine you're using will always use the correct pieces ( it couldn't use the wrong pieces, if it tried, anyway ).
Remember that most 'apps' ar just a special kind of folder. You can see what's in there by using control-clicking the 'app' and selecting "Show contents". You'll probably see at least two folders in there, one PPC and one Intel.
If you know you won't be using the same app on different hardware, you can use Monolingual to strip away the non-functional pieces in the packages. The space you save may not be worth the effort the way drive sizes are increasing.