We have this problem: Full and differential or cumulative backup have
approximately same size..

May be some application reset archive bit. In the log wee see
tar_base::V_tarMsgM: WRN - Archive bits will NOT be clared

We use Netbackup 5.1 Mp3 on master server w2003 and nw6.5 sp2 with client
netbackup 5.1 mp3 for netware...