I think - and this is an educated guess - that a foreign vocabulary takes up the same space as a native vocabulary and that if you hold everything else equal, I'll know more words in my one language than you will in that same language for the simple fact that you have another language taking up space.

Is human memory analogous to a hard drive, though? In regards to long-term memory, I don't believe we really have a limited amount of 'space' that we can fill up (short-term memory could be a different thing entirely). Everything I've read lately has suggested that memory has less to do with individual cells holding data, and more with memory being a process that arises from the interactions between cells. With this in mind, I think how many words you can "store" has more to do with the techniques you use to memorize them than how much of your total memory has been used up.

Of course, all things equal, if you're putting the same amount of resources into memorizing two different languages then I'd assume you'd know less of each. However, I don't think this is as much due to an inherent limitation of the brain as it is due to a limitation of time and resources.