The interim "fix" of deleting the file with Netscape 7.x open, and after closing the file being regenerated at a significantly smaller size works; but once re-sized it is imperative that you make the file "read-only" ("right-click" on the file, select Properties from the context-menu, and tick the correct box, apply and close) to ensure it doesn't start growing again.
Incidentally, something that is not clear in the prior "posts", is that
this file should be re-generated from within each <user-profile>, with
each handled in the manner I outline.
Warren C. E. Austin
The file is the Netscape "Registry" that stores all of the profile
information, etc. It increases in size due to plugin update information
from what I'm told. I don't have a very complete analysis at hand
although the programmers are quite upset at the plugin people for some
reason or other ...

--
Jay Garcia - Netscape Champion
(For those who have multiple profiles, take note... when you
delete registry.dat, NS appears to only recognize one profile
when you restart the program. Open the Profile Manager and
create a new profile for each of the other profiles using exactly
the same names they had previously. It will not overwrite the
old ones, as one might expect... it will re-activate the ones you
are looking for.)
Laurence