Take a look at the lead image for this story: what do you think it was shot with? It’s a photo I use often here on the site. That photograph was shot with Kodak Portra with a Bronica ETRS. No editing was done. It looks like it could have been done with a modern full frame camera or some other digital camera, right? To be honest, I could have done it with 35mm, Micro Four Thirds, APS-C, etc. What really mattered was the lighting and the situation because the further truth is that the laws of exposure don’t change.
Here’s the absolute truth about sensor sizes and image quality: in the hands of a photographer that sits there and uses a camera for what it is, the camera will produce fantastic images. All dedicated cameras these days produce more than good enough image quality, but they all require you to do certain things to make their peak image quality really come out. The results from an APS-C sensor or a Four Thirds sensor can all product jaw dropping images.
The secret: it’s in you. The laws of exposure don’t change; but you should have an understanding of how the rules of depth of field, contrast, and colors interact with one another.