The displacement is based on the volume of the sub not the mass.
Basically the Alfa was a small and light sub with probably the &quot;biggest&quot; reactor in terms of power output for it&#x27;s size.
What you got is a sub with half the displacement of an LA class submarine with a reactor with equal power output which meant that you could strap on some really powerful engines.
So half the displacement with about the same power capabilities means pretty much that you can double your speed.