I don't think that balance is a factor. I-4 or flat 6, both should be capable of being perfectly balanced and the Porsche and Volvos should be so.

Static and dynamic balancing of a V-6 or V-8 is another thing, but not part of your question. For years the V-6 engine seemed attactive (power from a shorter engine) but it was thought that it could not be balanced.

Spinning weight, mass and inertia are an important part. How fast more combustion power in proper timing can be added. If you knew the spark advance curve of both engines, and maybe there is a fuel injection curve too, you would see some of this.

Consider displacement and maximum air volume through the intake system, too. I bet the Porsche has a more polished intake air pathway. We used to call that "ported and relieved", done to an engine for adding power. Making a ported and relieved engine is not compatible with mass production and competetive pricing.

Volvo is designed for a drive that is not the same as the Porsche design. Spark advance, fuel injectors, injection management, and cam grind (timing and valve lift) come very much into play.

Interesting question,

Bob

:>)