I'm no engine "guru", but I've already asked about this and found out why....it's simple. I imagine the 600's have lighterweight engine components because they're continuously being developed and made better and faster, whereas these little parallel twins haven't seen motor changes in a LONG time. I also think the 600's have a lighter flywheel... These parallel twins, because the cylinders are directly opposed, they make a lot of vibration and shudder and nasty stuff. So there's a lot of counterweighting and balancing weights slapped on to dampen them out. I-4's don't have those types of problems as much.
its physics, a twin has longer rods than an inline 4, (a longer lever) so a twin has that longer lever that which creates more torque but requires more time to move that lever throughout its range. go to 8thgradescience.com to learn more. LOL