Originally posted by Rondec I mentioned above, but a lot of people have the perception that a 200mm f2.8 lens on micro 4/3 is the same as a 400mm f2.8 lens on a K-1. It is nice to be able to explain why they aren't exactly the same lens. Because based on apertures and angle of view, micro four thirds kills cameras with bigger sensors. Except it doesn't really.
My response to that is this image....
You'd need a 1200mm lens on an FF or 800mm lens on APS-c, and I got this with a camera i can almost slip in my pocket.
In some ways, small sensors do "kill" larger sensors. You just have to understand the limitations, it doesn't kill on every image but it will on a few, and at times it won't even be useful, but when it does what it does, ya, it kills larger formats. Every format has it's strengths and weaknesses. Smaller formats tend to be less strengths and more weakness, but they still do well what they do well.
On a really bright day, (100 or 200 ISO) with a relatively stationary subject so you can use the 2 second delay on a tripod and a subject that needs some serious depth of field to keep it all in focus ) I'll take my XG-1. And there are other serious advantages, like being able to use the camera like a spotting scope, because it ranges from 4.5mm to 233mm. You zoom out and find your subject then slowly zoom in keeping the bird in frame. No searching the trees through a very narrow FoV trying to find your subject.
It isn't the same really, but sometimes it's better, in many ways, and while the IQ may not be the same, as long as it's good enough for the purpose intended, no one cares how much extra resolution you are throwing away when you downsize the image. That one is 16 MP reduced to 4, do I care if it was 36 MP reduced to 4? it would still be 4 MP.
When discussing equivalence you are actually discussing very little about these camera systems, and probably the least important. It's all about a theory with very little to offer apart from academic comparisons.