I've found a one-liner solution on ETH Zurich Department of Physics wiki page (close to the end of that page). Just do a git gc to remove stale junk, and then
git rev-list --objects --all \ | grep "$(git verify-pack -v .git/objects/pack/*.idx \ | sort -k 3 -n \ | tail -10 \ | awk '{print$1}')"will give you the 10 largest files in the repository.
There's also a lazier solution now available, GitExtensions now has a plugin that does this in UI (and handles history rewrites as well).
