The answer is probably quite complex and in some way may involve almost all factors mentioned plus several that we won't think of.

For instance compression. Very low compression will make an engine sluggish, but very high will as well. This is why diesel trucks have Jake brakes as they have so much compression that their cylinders act like air springs, and they do not slow down well, this also allows them to idle at very low GPM rates.

When you have opposed cylinders, the mass of the pistons does not travel as far from the center of the crankshaft as the piston mass travels on an inline engine, this gives it better acceleratiopn as will as deceleration.

It is likely that the Porsche tachometer has better response than the Volvo tach, making the engine appear more responsive.

Your 911 most certainly has less internal friction than your brick does, racing engines have short pistons with minimal skirts to reduce friction.

As for piston weight, I'd wager that the 6 pistons in the 911 weigh less than the 4 in your volvo. I found an ad for 911 pistons that say they weigh 417 grams each and have wire retainers and tapered wrist pins. I'll bet that the Volvo wrist pins are not tapered (heavier) and have clips instead of wire retainers.

The clutch and the input side of the transmission spin with the engine when you rev it at idle, are the Porsche parts lighter than the volvo's?

And on and on...

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'96 855R,'64 PV544 driver, '67 P1800 basket case, '95 855, '95 854, the first three are mine, heh, heh, 415,000 miles put on 9 bricks