IIRC the Subaru flywheels for the Impreza weigh in at about 20 lbs - and this is likely similar or identical to what is used in the Foresters.
When the clutch engagement time is say 1 second and your engine is idling at 900 rpm you're only doing 1.5 rev/sec. In a four stroke, four cylinder engine that means you're getting on average 1.5 combustion events per second. In the 1 second engagement, you get 1 or 2 combustion events. In the 8 cylinder you get at least 2 and probably 3.
I thought about doing the math for how much energy is in a cylinder firing versus stored in the rotational inertia of the moving parts, but honestly I don't know that I really care. I know anecdotally that big engines make it easier to get cars moving for some reason, and that smaller engines make it a little more difficult. I just speculated as to why I thought that might be.
With that said, check out where the engagement of your clutch is. I've had people complain about grabby clutches(stalling) only to find out the engagement point is almost at the top of the throw (Former Subaru tech)