Size of Log Backup is not reducing from past three days

Recently we implemented Incremental(Enabled allow incremental dumps) and Log Backup(Disabled trunc Log on chkpt) for Sybase ASE(15.7 SP139) running on HP-UX Itanium server and to our great surprise, Size of Log backup is increasing from past 3 days continuously. Even after Full/Cumulative backup, size of the Log file kept on increasing at disk and even size at database is not clearing. Checked logsize at different interval by running command sp_helpsegment logsegment within database and noticed that used_pages is constantly increasing and free_pages is on decline. We even checked dbcc gettrunc to get details however it was of no use. Below is size of files We are using 2 stripe for database dump and compress 5 for compressing size on disk while taking dump. Replication is running on this database and is working fine. Appreciate if I can get expert advise on this issue.