git gc --prune consumes *lots* of memory with repos cloned with --reference

The situation:

I cloned linux-2.6
Then I cloned netdev-2.6 with --reference $PWD/linux-2.6
Then I cloned net-2.6 with --reference $PWD/netdev-2.6

The problem:

git gc --prune on netdev-2.6 takes WAY more memory than git gc --prune
on linux-2.6
git gc --prune on net-2.6 takes WAY WAY more memory. 1.5G and counting.

Why does git gc --prune take 1.5G? (git pack-objects is the actual
memory-hungry process)

--
Jon