This is one that is almost inevitably put about by 4/3 devotees, who can be a very strange lot at times. However, the heart of the myth is that there’s little difference in physical size and further that the difference is eroded in practice due to cropping required to make the images from 3:2 format cameras fit standard paper sizes - one site claims APS-C is only effectively 14% bigger than 4/3.
So, let’s look at the facts, shall we?
The dimensions of Canon’s APS-C sensors are 22.2 x 14.8 mm - that’s 328.56 sq mm. (I use Canon for a comparison as the dimensions of the imaging area are known - Nikon quote total sensor dimensions.)
The dimensions of the 4/3 sensor are 17.3 x 13.0 mm - or 224.8 sq mm.
So the difference is 103.66 sq mm, which means that the APS-C sensor is 46% bigger than the 4/3 one.
Now, let’s look at common paper sizes.
6″x4″ plainly is in a 3:2 ratio. 7″x5″? Well, that’s 2.8:2 or 4.2:3, so cropping on either format.
10″x8″? 2.5:2, or 3.75:3. A4? 2.83:2 or 4.24:3. And so on - it’s clear that there’s no magic advantage for 4/3 for popular sizes.
So it’s clear that unless you’re incapable of elementary maths, 4/3 suffers from a pretty significant size disadvantage compared to APS-C. Does that matter? Well, it means that for a given pixel count, the pixels have to be considerably smaller, leading inevitably to greater noise.