I remember the infinite thread discussion on what was a mainframe and don't want to go there again.
Call it what you want. Everyone (personal and corporate) has a definition of what is "mainframe", "midrange" and "personal".
Here, in just this data center, we have a IBM 3090 (?) mainframe with 36tb of data.
And a "midrange" (over 500 Sun boxes) with 98tb of SAN data.
And over 1400 NT (2003, 2000, NT) boxes with and unknown amount of storage.
How do I know what's a mainframe? The boss told me. ;-)