Database snapshots are read only. Completely
That said, it's highly unlikely that the snapshot is taking the full 25GB. Snapshots use sparse file technology where the file is only as big as the data in it, not the size of the source database.
Check the properties of the file and check the 'size on disk'. That's how much space it's actually taking up.
Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability