Its really not as simple as 2 or more engine sizes that share the same basic case where more displacement is better. For example, The ST GS40 loves higher RPMs and really comes alive when allowed to turn up .... but the 45 in the same case is more of a torque engine and likes larger props turning slower than one might expect. Both are great engines but with very different personalities. The same situation exists comparing the ST 61, 75 and 91 within the same basic case size. The 61 and 75 thrive at higher RPMs where the 91 likes a fairly heavy prop load. Same again for some of the OS 40 / 46 engines and a bunch of other engines that share the same case family ... I just used ST as an example because they are still being produced. The cylinder port timing can vary considerably between these engines making them have very different personalities. One engine in the family may be considered a "performance" engine where another might be just a "sport" engine. You have to evaluate every engine for its performance potential and whether it fits your application? Maybe the larger prop required for the larger displacement engine won't allow for enough ground clearance if mounted in a plane designed around the smaller engine. You get the picture here ..... larger is not always better. If a high performance, smooth running, well mannered .40 has more than enough performance in a plane why would you want to put a 45, 46 or 50 in there. Conversely, maybe the larger engine is the better choice and is the better engine .... you get the idea here, choose the engine for the application not by its size.

Then there is the matter of vibration. Larger displacement engines almost always have more vibration than smaller ones and I have seen some large displacement engines in small cases get the reputation for being "shakers"! Again, not always but its still something to be aware of. Maybe the larger engine is smoother .... it happens from time to time.

Because this is a "gas engine" forum, an example of two gas engines that share a very similar size case size and weight (not identical) are the Zenoah G45 and G62. The 45 weighs almost the same as the 62 and the physical size is very similar. Why would anyone want a G45 if they are so similar in all respects including price? Well for one, even though the 62 is a great engine and far more powerful, it is a bit of a shaker compared to the 45. Add to this the 45 is an extremely well mannered engine, a real peach of an engine in my book. If the 45 has enough power for the application, I would choose it every time over the 62 because it is so well mannered and smooth running...... and yes, the 62 is a great engine any way you rate it. Again pick the engine size for the application not because it has more power.