If I'm using Gmail, can I clean out the space that "Mail Messages" are taking up on my hard drive? In looking to make some room on my MacBook Pro's Hard drive, I ran the excellent Disk Inventory X

As I expected, movies, my Aperture Archive and other media-intensive files took up a ton of my space. But I was surprised to find that files that Disk Inventory X categorized as "Mail Message" took up a whopping 22 GB.

I mostly use Apple Mail as my email client, although I've been giving Sparrow another try lately.  But in either case, other than a legacy MobileMe account that I never use, Gmail is my mail provider (and storage facility).   

What the heck is stored locally in that size, and can I safely delete it somehow?

(I can't fathom it matters, but I'm running Lion.)
Mail is caching your Gmail contents and this is normal for IMAP. See this SuperUser question and answers for possible solutions and implications regarding SpotLight search through emails:


Disable all message caching
Disable attachment caching