Because if you're on a slow or metered connection (which many people are, like using cheap mobile plans), loading up a forum page or image with 50 comments and each have a "large" unique avatar could mean quite a bit of bandwidth use.
300kb should be more than enough for a 500x500 JPEG; I did a quick check just now and a 2048x1536 screencap like this>>2903740 resaved as a JPEG with minimal compression comes down to _just over_ 300kb.