Hi Nag,
For Key Figures of type NUM (Number) Domain RSKYFNUM is used. It has data type DEC with 17 characters and 3 decimal places. This is the standard system behavior.
You are able to go around that by defining a custom domain and replace the standard domain in table RSDKYF. The detailed procedure is described in SAP Note 460652 - Extending key figures in BW.
Best regards,
Sander