I checked out a project from svn... 

shenki@deadalus:~/src/bertos.svn
$ du -sh
14M     .

I also checked it out using git-svn. Here's how big the directory was part way through the initial git svn fetch (where it walks all the revisions and turns them into git commits): 

shenki@deadalus:~/src
$ cd ../bertos.git/
shenki@deadalus:~/src/bertos.git
$ du -sh
50M     .

And here it is once it's finished, and ran a git gc (garbage collect): 

shenki@deadalus:~/src/bertos.git
$ du -sh
8.0M    .

Keep in mind that the svn checkout contains only _one_ revision with some metadata (but no commit logs, that stuff has to be fetched from the sever when you 'svn log'), while the git repo is _every_ revision plus local commit logs.

So where's the space going? 

shenki@deadalus:~/src/bertos.svn
$ find . -name .svn | xargs du -sb | awk '{ total+=$1 } END { printf "%i KB\n",total/1024 }'
6130 KB

The total size of all .svn directories is 6130 KB.

For git 

shenki@deadalus:~/src/bertos.git/.git
$ du -sh
3.4M    .

This adds up, as a svn export of the tree gives us 4.7M.

git:

svn:

Yay for git's efficient packing!