I was so scared for you haha. You would have had to restart from 0! Unfortunately, nope. The size is what it is and there's no way to use custom compressed formats or models or anything, only the raw files. UTAU is simple to a fault
In order to stop this from becoming an issue, I personally keep all my UTAU voice banks in a folder on an external drive/partition/USB stick instead of using the \UTAU\singers folder. It's a little bit of a pain, but less of a pain then having to keep it tidy. It's also good to have the backups anyway, and it's not very hard to load them in manually.