In addition to the decreased weight, less moving parts means less internal friction.
Here are some interesting and potentially relevant articles and pictures that help explain what they may have done... how they achieved same or better revs with a piston that's 20% larger...
https://dustrunnersauto.com/what-is-a-stroker-motor/
Crankshaft designs have a lot of influence on power curves.
View: https://youtu.be/iX4byqM_IdQ?t=132
Note the two-up and two-down in close succession cylinder arrangement of a 1.6L short block...
View attachment 46132
Unable to find a 1.5L short block, this is a 1.0L...
View attachment 46134
Besides moving more air with a better turbo, perhaps inertial torque of the 3-pot crankshaft is a factor? That might have some bearing on the results of the rolling race. (before the 12 second over-boost limitation expires - the limitation presumably imposed for the cheaper cooling system) Torque is fun.