I usually render at either 800x800 or 1024x768. If I add a frame to the image prior to uploading it, the size will be a bit larger (less than 100 pixels) than those dimensions. I do realize that depending on the resolution of the viewers computer, the image may be larger or smaller than it is on my screen, but not unreasonably larger.

Many upload large images simply because they can.  The fact that the maximum gallery size is 4096x4096 doesn't help either. There are some people who  push the limits and will always choose the far end of the spectrum rather than a middle ground.  That whole "bigger is always better" mentality.

One time a friend of mine got some critique about her gallery images being too small (she worked with the default pose window and rendered at the same dimensions), so to try and appease her viewers she decided to render at the largest size allowed here thinking it was better.

When she sent me a link to her first large rendered image all I could see was the elephant's eye and I had to scroll around and around to see the rest of it,  LOL  I told her "Nice eye!" And suggested alternate render sizes to her :)

"It is good to see ourselves as others see us. Try as we may, we are never
able to know ourselves fully as we are, especially the evil side of us.
This we can do only if we are not angry with our critics but will take in good
heart whatever they might have to say." - Ghandi