I'm using BE 9.1 and have backed up some directories on a NW6sp5 box
traditional volume and have tried to restore them to a NSS NW 6.5sp3 box.

The restore seems to go well but the log is full of insufficient space
messages. The target has plenty of spare capacity, over 100gb spare. The
source also has a fair bit.

Compression was in use on the traditional volume so I told BE to decompress
before backup. This went fine.
There are space restrictions on the nds account, and the NSS volume does not
have compression.

Is it that my source restrictions are being applied by BE and then the data
cannot fully be restored ?

Can BE not restore the space restrictions ?