I can only imagine the instance admins having taken upload space into account, though I can’t say for sure regarding the individual instance.
If you don’t feel comfortable uploading to the instance itself, linking to other sources, such as peertube, should most definitely not require any significant amount of space.
Regarding autoplay of videos and content, I’m not sure how well Lemmy handles integration of outside sources. Though, it would only make sense to me if Lemmy natively supports Peertube videos or Pixelfed images, and if this is not the case, perhaps a Github ticket is required.
Regardless, I’d definitely also be interested in having a nice and integrated user experience, while not downing my own instance.