quality drops so much when getting many views

hello!

so i regularly post on the app, i share a lot of my photography. i always post them in a slideshow, but since my posts have been doing quite well for the type of content i post the quality seems to drop so much, but only on the posts that get like over 20k views. why is this and is there any way this can be changed? it's really annoying honestly because the pictures just look weird now...

thank you! :-)