It is nearly impossible to actually delete revisions from a repository, but if you can determine the actual revision where the size suddenly ballooned you can dump the repository out to a temporary file and load it into a new one.
Get onto the server where the SVN server is installed then:
svnadmin create newrepo
svnadmin dump -r 0:1000 > oldrepo.dump
svnadmin load newrepo < oldrepo.dump
mv repo oldrepo
mv newrepo repo
0:1000 denotes the start and end revisions that you want to restore/keep.
This SVN man page tells you how to make it work: Migrating Repository Data Elsewhere
Other than that there is no real easy way to simply nuke out one single revision.