Every time you do not define a domain when creating a column in a table, firebird creates a new domain.    Unless they have changed something fundamental, they never purge unused domains.   So, you perform a backup and it recreates all the prior domains first, then, proceeds to create new domains.   It also may have an internal counter that may not be reset to keep track of the next internal domain name to use (don't know, have not looked at the code for years).

Either way, you need to get your data out of the database and into one that is restorable.   You also need to change your create table routines to use pre-defined domains.   What you did not realize is that regardless of whether you want to use domains, you will be using them.   The difference is whether you use a domain you define, or let the system create domains on the fly.

At least with domains you create, you can reuse them and you do not encounter the issue you are seeing.

1 GB is not very much, My smallest DB that I maintain is 20 GB.  

With FB 3, you can do a few things to speed up the process, but, at this point, you may need more help than simple forum list support can give.    

Where on the planet are you located?   Perhaps we can get you in touch with a FB specialist in your area.

best regards

Dalton