The JavaDoc for the SizeRotatingFileHandler.setSuffix() indicates that files will not be deleted when deleted. However there are multiple rotations for the same date they will be purged.

For example if the suffix is .yyyy-DD-mm, the size was reached 20 times on the same day and the maxBackupIndex was set to 10 there will only be 10 files kept.What won't be purged is files from a previous day.