It’s easy to see the advantage of full frame! Except that most photographers don’t actually need 40mp of resolution, let alone 61mp. 61mp on full frame is more resolution than 40mp on APS-C, yet their pixels are similarly sized. Resolution is resolution, and 24mp on full frame is the same as 24mp on APS-C, yet the pixels on the APS-C will be smaller than those on the full frame sensor. Fewer pixels allows for better low-light capabilities and dynamic range, but at the expense of resolution. Smaller pixels will allow for increased resolution, but at the expense of low-light capabilities and dynamic range. There are two options: smaller pixels or fewer pixels.
Since APS-C sensors are smaller than full frame, there is less physical room for light sensitive sensor elements (pixels).
This is where the stigma originated that APS-C is not for those who are serious, and to an extent it unfortunately remains to this day, despite so many incredibly talented and successful photographers utilizing APS-C models. APS-C was more affordable and smaller, so it was popular with amateurs and enthusiasts on a budget. Back in the early days of digital, when dynamic range and noise control were much more critical than nowadays, full frame had a clear advantage, as one needed to squeeze the absolute most out of their files and full frame allowed that.
