From older Meta posts it seem to me that the file size limit was higher in the past, and linked to the limits allowed by imgur.
However, now the limit is 2 MB (or MiB), and imgur allows much more than that ("The maximum file size for non-animated images (think JPG, PNG, etc) is 20MB. PNG files over 5MB will be converted to JPEGs.")
Why is the limit set to 2 MB?
Personally, my workflow to quickly answer question often involve capturing all or parts of my screen content and paste it to the post. However, often 2 MB is not enough to hold a complex (i.e. low compresibility) image of the size of a normal screen, which is very unfortunate, and forces me to save the capture to a file, then open it and save it again with higher compression settings, which is fine but time consuming.
2 MB is a rather small value. Why not to increase it to, let's say 4 MB or 6 MB? That later value correspond more or less to the uncompressed size of a 1920x1080 color image, that is a fairly standard screen size. Of course, compression can be done on the server side, but allowing larger file sizes would make it much easier for the user. I think that we all agree that "copy & paste" are great tools to increase efficiency.
I want to make clear that I'm not asking for bigger images to be allowed in storage, just to allow the upload of larger images, and then do the compression and resizing if needed on the server side as imgur do for all free users. So, I guess it is possible to offload that task to imgur, that already automatically resizes and/or compress large images.
For example, this image, that I captured from my screen and pasted it on the image dialogue was rejected in Stack Exchange:
But then I went to imgur and pasted it in the upload dialogue there as a free anonymous user, it got uploaded without any problems and was also automatically compressed. In fact, if you go to the imgur page of the image, and download it you will find that is uses only 345 kB.
As another example, I uploaded to imgur as a free anonymous user an image of 8.6 MB and 4608x3456 pixels in size with no problem, but the version stored on imgur after the standard processing is only 919.8 kB and 3226x2419 pixels in size. So there was automatic resizing and compression.
Wouldn't that same behavior be desirable on Stack Exchange?
The automatic resizing and compressing does already happen in Stack Exchange uploads, the only difference is the arbitrary limit of 2 MB on the original file size. Why is that? Is there a bandwidth limit in the agreement between Stack Exchange and imgur? A CPU time limit?