So it is 51, are you sure it's not plus 5 with our filler nozzle being on the passenger side. Is that the same side with the other A-Class models. 80 would have been more like it as my E-Class had which was a lot more frugal than the A45s.
I think in Australia the larger fuel tank is in relation to more frugal A-Class versions, the same as the UK. The increased size to 51L comes in from the A250 4-matic file:///C:/Users/chas_/Downloads/a-class-hatchback-specifications.pdf
It certainly would be good to have a larger tank option. Such as 80L as I had on my E-Class. And I can't understand why MB dropped the size from 56L for the previous A45 when the A45s is no more frugal, at least the way most of us drive it.