Well, the backups would never be larger than the domains they’re backing up.
So you may need to limit the size of the domain itself.
Or if there’s one domain in particular that’s causing a problem, you could edit the backup schedule, and configure it not to back up that particular domain.
Now, you could configure the backup parameters to use additional compression, or bzip2 rather than gzip. That’s not likely to make a huge difference though, only a slight one.
Other than that, you’d probably need to look at either limiting the sizes of the domains, or not backing up the domain offsite.