To find out, just log in to RubyForge, go to your project, and click the Admin tab. The totals are split out for "SCM" (source code mgmt, e.g., cvs/git/svn), space used in your project's virtual host, and space used by the files you've released:

Here are the top 10 projects in terms of SCM usage:
gforge=> select g.unix_group_name, d.scm_space_used/1000
as MB from disk_usages d, groups g where g.group_id = d.group_id
order by d.scm_space_used desc limit 10;
unix_group_name | mb
-----------------+-----
blacklight | 598
cougar | 576
instantrails | 547
easygameengine | 249
ogrerb | 228
rubyes | 215
fxruby | 187
dojo-pkg | 177
restore | 174
tubix | 150
And for released files:
gforge=> select g.unix_group_name, d.released_files_space_used/1000
as MB from disk_usages d, groups g where g.group_id = d.group_id
order by d.released_files_space_used desc limit 10;
unix_group_name | mb
-----------------+------
backlog | 1130
instantrails | 960
rubyinstaller | 726
rhodes | 666
rmagick | 603
wxruby | 535
rails | 369
linnet | 326
rubricks | 323
fxruby | 316
And for virtual host space used:
gforge=> select g.unix_group_name, d.virtual_host_space_used/1000
as MB from disk_usages d, groups g where g.group_id = d.group_id
order by d.virtual_host_space_used desc limit 10;
unix_group_name | mb
-----------------+-----
instantrails | 328
funfx | 171
rubyworks | 116
fxruby | 109
roby | 97
scrubyt | 95
freeride | 91
twitter4r | 84
rpa-base | 76
rubyhackerblog | 74
Not surprising that InstantRails would be the leader or close to it in all three categories, I guess; such is the way of projects with large binaries. "backlog" has a bunch of war files - targetting JRuby, I reckon.
This information is populated via a cronjob, so if you clean up some stuff it'll be a while before the numbers get updated. Right now I've got it scheduled to run once a week. Mechanics-wise, it's very low-ceremony - it just iterates over the active projects and runs du -sk
on various directories.