I'm not an expert, but I feel what it is doing is correct. Now, whether or not it should do it this drastically with only a 2" difference in diameter, is another story. But it's not just a brightness and material thickness thing, there is a distance component that is part of it. Coming from the sign industry, it takes way more LED strips to light a 10'x20' sign cabinet than it does a 4'x8' cabinet. There's something with distance, falloff and spread that all get put into the calculation. You're also drastically increasing the amount of material area that the same light source illuminates - just the surface area change from the 3" to the 5" is almost triple (113.1 versus 314.16). More mathematical minds than my own could figure out the actual volume given the thickness of the globe (I'm no mathlete). In any case, you should expect some dimming from the same light source as the diameter of the globe increases. Maybe someone else can better input on the real world values versus the calculated ones in Keyshot. My suspicion is that if you took a camera and captured an image of the real-world items in such a way as to minimize the amount of glow that it would look more similar to what Keyshot is showing. If you want it to look more like what your eyes perceive, then fudge it in keyshot and eyeball the results till you get the look you're looking for. There may be a more complex way to calculate it, but trial and error might be quicker/easier.