When you create a new topic in the Deluxe Edition Dragon NaturallySpeaking, you are given the option to copy any of your existing topics or any of the base vocabularies which are shipped with Dragon NaturallySpeaking.  The current version of Dragon NaturallySpeaking Deluxe Edition includes three base vocabularies, a 30,000 word general English vocabulary, a 45,000 word general English vocabulary, and a 55,000 word general English vocabulary.  The 55,000 word general English vocabulary also has an expanded language model which increases accuracy slightly.

If you create a topic based on the 55,000 word general English base vocabulary, for example, then that topic will always have 55,000 words available to it even after running the Topic Builder or the Topic Editor.  The same applies to the 45,000 word and 30,000 word general English vocabularies (which set the vocabulary size to 45,000 words and 30,000 words respectively).

Having larger vocabulary allows you to have more words active a given time.  This is particularly important if you're dealing with a professional specialty like medicine or law and you are using a large vocabulary.  However, for most users of Dragon NaturallySpeaking, 30,000 words is more than enough and increasing the number of words does increase the potential for misrecognitions.

In addition, the 45,000 word vocabulary and topics derived from it require additional 3 megabytes of memory and the 55,000 word vocabulary and topics derived from it required an additional 5.5 megabytes of memory.

Do the large vocabulary sizes increase accuracy?  I believe that the jury is still out on this.  Clearly one of the most significant sources of errors during dictation comes from words which you dictate which are not in the active vocabulary.  But running the Topic Builder is the best way to minimize out of vocabulary words and increasing the size of your active vocabulary only helps if you regularly use so many words that 30,000 is not enough.