OH AL,
You are the hero again mate. Every time you will hit the nail in a single shot. You are awsome. I forgot this point and checking every where, SBIW, rcust6, package buffer, memory settings etc.
When you said, I recalled that as its test data ( IN DEV) and when the semantic grouping is active. Its actually putting all the related records in 1 packet size which is overriding the quoted packet size given in DTP or the SBIW settings. When I removed the semantic keys, all the packets came in 20000 size. You are awesome mate. Excellent job. I really appreciate your presence of mind.
For those who have landed up here searching the error in SDN. Please uncheck semantic grouping to fix this. Its mainly useful if you have error's while loading. So if you notice any error, you can enable it and create error DTP which can be fixed. if the data is relevant like in PROD data then you can leave the semantic grouping enabled. Hope it helps. Cheers.
Thanks a lot.