I am also not sure where the size estimates are coming from.

At a minimum you would need to store the account # (64 bits) plus the public key (256 bits) plus the balance (64 bits?) which would be 384 bits or 48 bytes per account.

If this were being stored in a "flat file" then 1M accounts would requre 48 MB (and 1B would require 48GB).

Assuming only a "flat file" was used (i.e. no index file) then a map (stored in RAM) to identify where to find each account (64 bit file offset) would be required (if we had 1B accounts then that map would easily be at least 2GB and although that isn't too unreasonable it would make starting up the node very slow as it would need to create that map by scanning a 48GB file).

More likely you'd use an index file then (so add 2-4GB extra) requiring a node that is handling 1B NXT accounts to allow for approximately 50-52GB (actually not too bad IMO) for the "last checkpoint".

Assuming that the "blockchain" between checkpoints is no greater than say 50GB then it would look as though 100GB would probably be enough (although this last calculation would need some more analysis).