It's probably using something like TOP msg 0 to download the header block.
My POP3 knowledge is limited, so I guess that's POP3's equivalent to IMAPs capability to download header blocks I guess.
In which case my original statement still stands. It's a lousy method compared to sever-side.
You're still eating up your bandwidth and processing power. And parsing headers gives you a very limited anti-spam coverage, and
NO anti-virus coverage (since viruses are in the message body, obviously !).