The british civity's are not the same as the spec we are getting. The NSW trains are wider for a start and I also suspect taller. The 22 litre easily fits underneath the floor and it isn't being shoe horned in as you suggest. All up it weighs less then the prime mover and generator in a single xplorer/vlocity unit. In fact I think it might even be lighter then the qsk19 by itself.

Check out the hybrid power pack section at this link. https://www.mtu-solutions.com/na/en/applications/rail/railcar-powerpacks.html

I'm not saying the Vlo's are slow but electric trains always accelerate faster and these trains are electric trains with a diesel generator and batteries.