The bigger engine should not be accommodated for a stronger chassis? How is this even remotely logical?
The bigger engine also brings in tonnes of additional baggage (CDI, wiring harness, regulator/rectifier, a working stator, carburetors) and fiddly chassis modifications for engine mounts, that would render the chassis damaged/useless to a certain irreversible extent.
Consider examining the GS500E's chassis with a GS150's, and then compare a GS150 engines weight to a GS500E's. The difference should become relatively obvious.