I would like to know why the forum is creating 24-bit PNG thumbnails that are ofter bigger in file size than the actual image!

While PNG is great for most screenshots, is not well suited for thumbnails as text over a single color/grandient becomes a bunch of different grey pixel that are harder to compress. As a thumbnail usually doesn't need 16 millons colors and lossless compression, a 8-bit (256 colors) PNG or a JPG with moderated compression may be better.