When HL1 was out, my maps folder was like 4 gigs. Yeah it had a s**tload of mapping crap in there but a fair portion was good old .BSP's.
Maybe it's the current lack of a large reviewing website for HL2DM? I too remember having my HL1 maps dir being gigabytes in size...and got almost all of them from the old Radium reviewing website.
Whilst it is possible to jump on a new server and try out some new HL2DM maps, I'm not particularly keen on this myself as I've been biten a number of times wasting time encountering poor levels and moving from server to server, then having to trawl through my maps dir and delete all the chaff.



