Time machine backup grows while backing up until runs out of space

Ever since I installed Mojave my time machine backup grows out of bounds and fails when it runs out of space. When I fist start a backup on an empty newly erased disk, it tells me that it is backing up XGB of 502GB. Sometime near the end of the backup the XGB of YGB starts growing - both!! And the difference is about 70GB. That difference stays constant until there is no more room on the drive and the backup fails. The disk I am using is a 1TB Seagate USB drive. I have Mojave 10.14.1 installed. This also happened with 10.14. Help!