I think I know what you mean:
When you upload it to Mega, the weight of the file remains original (therefore the quality does not drop or anything strange). When you publish it directly to pages like Kemono, Rule34, Newgrounds, etc., the website compresses it, reducing the quality and weight of the file. Sometimes it is very noticeable and other times it is almost undetectable, in the case of Kemono there is not much difference, but yes, that video is already compressed. That's why it's always better to share it on Mega, Pixeldrain, Bunkr, etc., so you don't run the risk of losing quality.