I've had the chance to join several big servers and even manage my own (calckey.social). Usually, they set an upload size limit of around 10-15MB, which is more than sufficient for most photos.
However, I must say that I'm not a big fan of Imgur. It's not the best experience for visitors and feels like a prime example of something being unnecessarily unpleasant. I believe people mainly use Imgur out of habit, and the drawbacks haven't been significant enough to overcome that familiarity just yet.