Linux or Unix Incremental backups are as big as the full backups, all the selections sets and backup option sets are good, everything seems to be working in order. Checking in the trace logs and binary logs, it genuinely thinks that all the files need to be backed up.
Investigation reveals that the saveset was backing up a volume on automount, but when defining the backup set, only the folder was selected without expanding it. Due to this, the automounting did not happen and the folder was seen as a simple folder. When the backup runs, the folder is accessed and the automount happens. As such the folder definition went from simple folder to mountpoint and as designed, everything in there was backed up again.
A good way to see if it is an automount folder, is to open the folder before selecting it. This will force the automount to work, and you should see the icon of the folder changing for simply yellow to a yellow folder having a small black icon at the bottom right of the folder.
In this case the parent folder was the mountpoint, but it can be that the mountpoint is further down the hierarchy. This would be the case if the incremental backup would work for certain folders but not for others, then ivestigate if there are any automount mountpoints in there.
NB. tools like checking fstab and running mount might not show possible automount points as they are not permanently mounted and usually it is because of a third party software that they get mounted.
An alternative approach is to select a directory above the automount point and configure "backup throught remote mounts". Any automounts or directories that are not required to be backed up can be excluded by deseleting them.