A quick comparison of current Euro6 engines from Volvo http://www.volvobuses.com/bus/na/en-us/_...mId=130637&News.Language=en-gb

Looking ay the discussion re engine size and bhp - my last two cars have been 1.6 and 1.9 respectively. One a Ford engine, the other a VW.
Both put out a similar bhp, emissions, fuel economy etc and both cars are of a similar size/design.

When humping them up and down the motorway, usually at a similar sort of speed, the 1.6 version needs to work harder than the 1.9 by virtue of its design and size.

If you compare the B7 to a B9, are you not going to have the same issues?
If two cars are performing different and there is just under a 1/3 of a litre between them, what sort of difference will there be between two buses which have a 2litre variance?