First, let’s look at pixel counts on different size sensors.
Suppose we had a 40MP APS-C sensor – would that produce images as good as a 20MP full frame sensor?
Well, noise for a given ISO is lower on a bigger sensor because bigger sensors gather more light. So it is not automatic that a bigger APS-C sensor would outperform a more modest pixel count on a full frame sensor.
Small sensors also mean more depth of field for a given framing, so to get the same subject separation you would need a shorter focal length or you would need to move back further from the subject.
With those two points out of the way, the question is whether more pixels on a sensor is better than fewer pixels on the same size sensor. And by same size I mean that same physical dimensions, in this case APS-C.
On the principle that a picture is worth a thousand words, here is the full frame from the Fuji X-T50 with it 40MP APS-C sensor, and a crop of about 9% of the full frame.
The full frame is 7728 x 5152 px. I downscaled it to 1000 x 1500 px to display here. The crop is 1622 x 2433 px, also downscaled to 1000 x 1500 px
One thing I know is that the crop is much more detailed than I could get from a lower pixel camera with the same size sensor.
And to show you what that deterioration in quality actually means, here is a smaller crop upsized to 1000 x 1500 px.
