There are exceptions, as my sibling comment points out, but if you go to a list of the top macro photographers and check each one out, 90+% of them will be using 35mm cameras.
Instead, most people come up with all sorts of justifications for why their equipment is the best. And that’s ok. In the end, photographers take good pictures and we are happy. There is no underlying need for photographers to use the “best” equipment available to them, and in actual practice, you will find photos in galleries taken using a camera phone or a cheap plastic Holga, hung right next to pictures taken on a 4x5" that look immaculate.
> Perhaps you get some advantage of more depth of field, but 35mm camera lenses can be stopped down to achieve the same effect, when it is desired.
The premise I’m operating under is that you’ll make, more or less, the same artistic choices with different equipment. In other words, if you shoot micro four thirds and switch to 35mm, you double the ƒ-number (e.g. go from ƒ/11 to ƒ/22), get roughly the same depth of field, roughly the same field of view, roughly the same image sharpness.
There are two problems here. One problem is that the image is not particularly sharp to begin with, so any extra resolution provided by a better sensor is wasted. The second problem is that larger lenses are operated farther from their ideal focal distances.
The formula for focal distance of an ideal lens is
1/ƒ = 1/d1 + 1/d2,
where ƒ is the focal length, d1 is the subject distance, and d2 is the distance to the image. Normally, d1 >> d2, and you can consider a 50mm lens on a 35mm camera as the same as a 25mm lens on micro four thirds. For macrophotography, d1 ≈ d2, and changing the sensor size has a radical impact on how far a lens must be moved to remain in focus in relative terms. That is, when you double the sensor size, you have to more than double the distance you move the lens.
This means that larger lenses are operating farther outside their ideal focal distances, and this has a negative impact on image sharpness. Macro lenses are specifically designed to be operated in this range, but the fact remains, larger lenses are still operating in a more extreme regime compared to smaller lenses.
You get pretty good results by just copying what most people use, but the fact than 90+% of people are using equipment X does not mean that equipment X is ideal. Plenty of musicians have completely incorrect notions about what kind of impact various choices they make about their gear has on how they sound, too—these beliefs often completely fail to hold up under any reasonable amount of scrutiny.
Photographers commonly believe that larger lenses take better pictures. Reality is more complicated—there are a ton of tradeoffs both with lens design from a practical perspective and tradeoffs involving the underlying physics that make the situation much more complicated.
The long story short is that reducing the aperture has two effects on image sharpness. Smaller apertures increase the depth of field, bringing more of your scene into focus. Smaller apertures also introduce diffusion, which makes the whole image softer. This tradeoff means that there is a medium aperture which makes your image as sharp as possible.
Photographing people with a 35mm camera, the aperture which maximizes sharpness is often achieved with apertures somewhere around ƒ/5.6 or ƒ/8. It is advantageous to use a 35mm camera because the larger sensor size can reveal more detail and capture more light. 35mm also gives you access to larger apertures and those nice, blurry backgrounds.
In macrophotography, large cameras just aren’t as advantageous. The small apertures that you typically use will produce soft images, and the soft images can be captured well enough on a small sensor. Small cameras give you very blurry backgrounds to begin with, so you wouldn’t choose a large camera to blur the background. Additionally, small cameras require less lens movement, proportional to their size, to focus at the short distances required for macrophotography.
I do some macrophotography with my RZ67. Mostly pictures of flowers. It’s fun, but I know that it would be much more convenient to use a smaller camera, and I wouldn’t be sacrificing image quality. One of the big reasons I use the RZ67 is because it uses a bellows to focus, and the bellows is long enough that you can focus very close. However, there’s a lower limit to the size of the subjects I can comfortably photograph, and if I were serious about macrophotography, I would use a smaller camera.
Summary: Large cameras are better at taking pictures of large things, small cameras are better at taking pictures of small things.