It's not necessarily a difference in size, but more on shape. BO basses need a neck pockett... for the bolts And they need to fit the std 24 frets Warwick necks.
A NT thumb has 26 frets, so the body should be a little bit longer for a 24 frets neck like the Bo.
Same with the Buzzard: 24 frets for the BO against 26 for the NT.
I think it's because I looked at pics of Entwistle too much, but I always thought the looks of the BO as being not quite right. Same with the headstock. The NT just looks better balanced, slimmer, faster, better.
But of course the BO still is very pretty and a dream to play and have.