Why Is My Database Running Out Of Space Already!! (urgent)

I have a question on database size, specifically, what difference would I expect in the size of a db (the space actually used for data, not indexes) if I have one single data device vs. multiple data devices spanned across different physical drives. I have 2 customers that have identical database tables (all columns are integers), customer A has 27 million rows and customer B has 36 million rows. If I do an sp_spaceused on the table, customer A's data takes up 9.7 gig, whereas customer B only takes up 3.2 gig, even though it has 9 million more rows. Statistics have been updated, so I'm confident the sp_spaceused is accurate on both databases.

The only differences I can see between the 2 databases is that customer B has 5 separate data files (6 gig each) for the database which are part of the Primary filegroup. These files are all on the D: drive, which is a partition on a RAID 5 array. Customer A has a single large data file (24 gig), which is also on a RAID 5 array.

Any ideas?