Cumulative Backups:

Start RMAN with a level 0 cold backup and then last night should have been a level 1 cumulative hot backup
however I feel that as this backups was taken without the level 0 cold backup The level1 hot cumulative was actually the equivalent to a full hot backup hence the size.
See example below for proposed solution
Level 0 is Full Cold backup (application down)
Level 1 is Hot Cumulative backup (application up)
Restores have not been tested until we create an RMAN duplicate on the test environment to verify the backups are complete by using the LIVE backups to create a clone of the system in the test arena.