Why the photo I posted in full image on sina Weibo turns out to be not as clear as it should be?

To avoid large images (especially photos from mobiles), most websites will resize your image on-the-fly when you post it.

The idea is that they don’t want to store and force users to download large images (usually <500k) so they modify it to have an appropriate size (likely >200k). That is why quality appear downgraded once you have uploaded it.

It is not specific to Weibo, most websites do that

This text was originally published in quora.