When comparing similar engines, the power is proportional to the cylinder volume, e.g., the power output of a .72 engine is approx 72/56=1.3 times higher than that of a .56 engine. Similarly a .56 can be expected to be 56/52=1.08 times more powerful than a .52 engine. You have to decide for yourself if the power ratio is significant or not.

When comparing four-stroke and two-stroke engines, things becomes a little bit more difficult because the power/rpm relationship often differs quite a lot. Usually a two-stroke engine will have a higher power output than a four-stroke engine of the same capacity, BUT the power is often produced at much higher rpm.
A typical two-stroke engine will develop its max power at around 15000 rpm whereas for a four-stroke engine the max power output is somewhere round 10000 rpm (the values are for the example only, the may differ significantly for different engines).

If the engines are loaded with propellers that give static rpms around 10000 rpm, the four-stroke will produce its maximum power whereas the two-stroke will be operating at a power output significantly less than than its maximum. How much less differs from engine to engine as the power/rpm curves may vary quite a bit depending on the engine design.

In my experience, with a big diameter propeller that keeps the rpm around 10000 rpm (or below), a four-stroke is about as powerful as a two-stroke of the same capacity. A OS 61FX, which is a rather powerful engine (1.9 bhp at 16000 rpm), will probably be a bit stronger than a Saito 56 when propped with big props such a 13x6 or similar. If a smaller prop is used, it will be significantly stronger.

Some two-strokes are designed for low rpm operation such as the MVVS 15 GFS/R. It produces about 2.4 bhp at 11000 rpm and will turn a 16x6 APC propeller at about the same rpm as a Saito 1.25 will do, approx. 9000 rpm. The MVVS is a .90 engine in a .60 housing and is happy with propellers between 14x7 to 15x10.

/Red B.