Top strap cutting and forcing cone cracking are different animals, but ... is either affected by the frame size?
this is a "No" and "Yes" kind of thing.
Both are more affected by the specifics of the ammunition being fired more than any other single factor.
Top Strap flame cutting.....
PERHAPS you might make the case frame size matters, but only if the distance from the upper edge of the cylinder throat (not the barrel) to the topstrap at the cylinder gap is different (larger) due to a larger frame. The distance gas has to travel from where it escapes confinement (upper edge of the bore/cylinder throat to reach the topstrap.
Remember that the gas is confined until the bullet base exits the front of the cylinder. At that point, most of the bullet is in the forcing cone (sealing it), and when the base of the bullet exits the cylinder the gas vents (radially) with a portion of it striking the underside of the topstrap of the frame.
This jet of very hot, very high pressure gas can, over time (number of rounds) erode the steel it strikes, to a point. Now, here's the kicker, how many rounds and how deeply the gas will, over time, cut, depends entirely on the ammunition being fired.
Perhaps the most famous example is the S&W Model 19 in police use. Lots of people have heard the stories about how the hot 125gr JHP ammo caused "excessive" flame cutting and forcing cone cracking. What usually doesn't get told is that those same revolvers gave quite a few years of entirely satisfactory service without undue wear with the then standard 158gr .357 Magnum ammunition, and that the problems only showed up after the standard ammo was changed to the 125gr stuff.
Additionally, its not like every model 19 failed. It was, literally very few guns, however the failure RATE was above expected and normal, so it was an issue. (for illustration, if your expected failure rate is 3 out of 1,000 and now you've got 6 out of 1,000 failing, its only a tiny number of guns, but the change in rate (double in this example) is a serious issue.)
Topstrap flame cutting is self limiting, even with the hottest most "cutting" ammo. Eventually it stops, when the metal being eroded away becomes too far away from the gas for the gas to be able to continue cutting. It is a noticeable groove, but not enough to affect the practical strength of the topstrap.
Forcing cones, on the other hand get the full brunt of the gas, and in the case of the S&W M19 the barrel forcing cone has a flat spot on it, making it thinner at that point than the rest of the barrel. This MIGHT account for the issue they had, but so would a leaded barrel forcing cone.
The barrel, including the forcing cone is a "wearing part". Meaning it gets worn with use, and eventually will wear out and fail. Many factors will affect the rate of wear, and its service life.
A kaboom has to explode the cylinder before the frame strength comes into play.
yes, a kaboom, does that in a revolver. However, frame strength is in constant play with every round fired. Think in terms of stretching and of holding the cylinder in proper alignment. Look at top break revolvers. NONE are made in magnum calibers. Modern steel will handle the pressure, right?
Yes, the steel will handle it, kind of, but the design won't. Cost and hand fitting (more cost) aside, there is simply no way to make the hinged frame and its latch mechanism able to take magnum pressures and provide acceptable service life in a normal size handgun. Absolutely adequate and safe for black powder pressure levels, but when you double or triple that things stretch that shouldn't stretch, which leads to battering with leads rapidly to unserviceable and unsafe.
EVERY frame of every size is built with more than enough "strength" to maintain proper dimensions during normal use. If you start running pressures above that, the larger frame (and cylinder) with more metal in place have a greater "reserve" than smaller ones.
So, its a yes and no thing. With certain loads, larger frame guns are able to "soak up" excessive force better than smaller ones. With most ammo smaller guns are more than sufficiently strong.
With all the different factors that are involved, looking at frame size alone isn't looking at things correctly.
clear as mud, yet?