That's a fair question.

I don't know that I can prove it 100%. What I can say, though, is that I can monitor disk space for the better part of 15 minutes and not see it change. Then I run a query such as SELECT TOP 500 FROM {table} and watch the available disk space get smaller and smaller. Space continues to decrease until either I stop the query or execution completes. The disk space is stable again (unchanged for several hours).