Forget the price of fuel. Does gas weigh more than glow? And does gas have more power than glow ?

Glow engines are more powerful. To be precise an engine running on a methanol + nitromethane fuel will give more power than the same engine converted to spark ignition and running on gas [ petrol ]

Petrol 737.22 Kg / m3

Methanol 791.30 Kg / m3

So methanol weighs a little more than petrol but that is only part of the story as an engine running on methanol uses a lot more fuel than the same engine running on petrol so you need a bigger tank.

In my experience, glow is better for smaller engines, say 1.40 and under. (That is a very general rule in my opinion but not set in stone) Without all the extra ignition parts, glow engines are lighter and make more power per a given displacment running on methanol/nitro-methane than gasoline. So I usually would never consider trying to use a gasoline engine on a .46 or .60 size airplane. I've seen people bolt gas engines on some planes and all the extra engine weight and work to acheive a good CG made for a poor flying pig of an airplane.

As engines and planes get larger though, gasoline begins to have a clear advantage. The engines can be priced the same or even less than larger glow engines. The wieght usually isn't a factor with the larger engines and airframes. The gas engines make great power for their size. Gasoline is less expensive and more convinient than glow fuel. Gasoline engines are also generally very consistent and reliable. I would almost never consider a 1.60 or larger glow unless it was just the absolute best option for the application.

Like I said, that is all a general rule of thumb. There are always exceptions. You'll run into some smaller airplanes that have big, round cowls and sometimes require extra nose weight to acheive proper CG. Some biplanes are an example of this. In those cases, that gasoline engine might be just the thing. There are also smaller and lighter gasoline engines coming onto the market. Magnum is even releasing a gasoline version of their .52 2-stroke glow engine. It sounds interesting but there will be the added weight of the ignition and extra battery pack. I'm sure it wil lbe a good fit for some airplanes though.

So a 50 cc size gas , makes sense.  And a 20 cc not a good idea.

Well, it depends. There are always exceptions to the rule.

The 20cc would make perfect sense on something like a Great Planes Stearman. There is plenty of room to mount it and the plane needs the nose weight. The 20cc would fly that airplane very well. It might even work good on certain warbirds with a nice round cowl. Iwouldn't try to put it on a .60 size trainer or your average .60 size sport plane though.

The 20cc might also work well on something like a very large trainer, large Decathalon or giant scale Cub too.

Pretty much. In the real world it's more like after a 1.20 glow you would want to go to one of the nice new 26 or 30cc gas engines like CRRC or DLE. But it depends on the plane too, like Chuck said, nothing etched in stone and not all gas engines are created the same. I still have one of the old RC Showcase 1.40 22cc gas engines that has more power then my YS 1.40 by a long shot. The engine is a rear induction high performance engine but no longer imported. It was also very expensive compared to todays imports from China. The DLE is pretty much a clone to a DA. There are now some small gas engines to be used in some 60 size planes but I have never used one or even seen one at the field. The 20cc Zinoah was designed for planes like the Cubs but I have seen one in a Mustang. There is on paper thinking and the real world, in the real world gas power is after 1.20 or 1.40, my stop point is 1.20. If you go into the gas engine forum you can read about what people have to say about the new little gas engines to get a better idea of what these new engines will do and what users of them think. Right now there aren't a lot of users yet.