Looking at:

http://www.secularhumanism.org/fi/dealing-with-dying/hoffman.jpg

...it shows lots of small freezers on wheels - and not one big one.

So, perhaps there are factors involved which have not been considered in this analysis. It would seem that the benefits of large size run into diminishing returns, and the costs rise faster. It is the same reason why there are no surviving land animals bigger than an elephant.