Holy crap! 108 MB is 90% of an entire OWRT partition I build on (120MB)! Even 53 MB is still too massive. Are you backing up any block lists? I'd rather download a fresh set after restoring a backup than include those lists in it — they'd typically be stale given how often one rarely restores a tarball anyway.
To give you an idea of the differences: without AGH but using GL GUI/firmware & numerous VPN conf, my Slate AX v4.6.8-beta9's tarballs aren't even 1/2 a MB; they're < 95 KB. On 'pure' OWRT with a similar setup for the same functionality that tarball is 82.2 KB.
Wow. I never knew. That's brutal!