I am trying to Detach an existing content db from my sql server by deleting its .mdf, .ldf files and while I am trying to add new content db (which I got from other server) whose size is 50 GB my sql server shows me error indicating that It needs a size of 137 Gb. I am not sure why it needs 137 GB of space rather than 50Gb. I think its asking so much space because to provide space for both mdf and Ldf file, I am I correct ? else please suggest.
Probably the log files are huge. Try Shrinking the database before backing up, and restoring.
MSDN about Shrink Database: http://msdn.microsoft.com/en-us/library/ms189035(v=sql.105).aspx