Not every bus has the same capacity...
Like for example, SBST & SMRT single deck Mercs have a capacity of 11967cc while DDs like the latest B9TL only have a 9.4L engine!

For SMRT Buses, all are over 11L I believe, except the Lances.
SBST got a wider range, from 6L for the Darts to 12L for the Mercedes.