I reduced the size of my backups by deleting my email account then deleting /home/user/.modest because I discovered modest was abandoning old emails files and clogging up its cache with files that should have been deleted!

Also clearing you browser cache helps, quite why the backup app will backup your browser cache is beyond me! I find it will do this if you have a browser extension installed.

Doing this has got me down from 32M to 8.9M

Hope this helps as 70Mb is a joke.