Mariabackup performed using incremental backup on tables using the myisam engine don't backup the incremental information. They back up all information. Here is the difference between backups with the same data on myisam (first set) and innodb (second set) tables:

The issue with the large backup do appear to be engine related. With a static DB I ran a full and two incrementals:
root@ubuntu-server-template:/opt/mysql/backup# du -sh *
34M base_2020-08-27
33M inc_2020-08-27_1
33M inc_2020-08-27_2

I then converted MasterImageSql from MyISAM to InnoDB. I cleared the old backups and ran a full and two incrementals:
root@ubuntu-server-template:/opt/mysql/backup# du -sh *
45M base_2020-08-27
1.4M inc_2020-08-27_1
1.4M inc_2020-08-27_2

Backup commands were run using identical command line options. On both, no new transactions were committed between the time of the full backup, and the time of the incremental backup

Full command: mariabackup --backup --target-dir=/opt/mysql/backup/base_2020-08-27 --databases-exclude= --user=root --compress
Incremental command: mariabackup --backup --target-dir=/opt/mysql/backup/inc_2020-08-27_1 --incremental-basedir=/opt/mysql/backup/base_2020-08-27 --compress --databases-exclude= --user=root