Hi, thanks for your feedback! The thumbnail urls that Lemmy returns via the API are actually the same as the image itself. For example taking the top post on my all frontpage right now I see a post with this image https://lemmy.ml/pictrs/image/f9c3fb6d-d7ad-4d8e-b0d6-f235a4497ca8.jpeg which is 2800x2707 and my network transferred 924.43 kB. That post via the api also has a thumbnail which is what Connect uses https://lemmy.ml/pictrs/image/40c8d517-c165-4be6-837f-a0385cf66ed1.jpeg however that thumbnail is also 2800x2707 and 924.43 kB. That said there are definitely optimizations we could be making since pictrs does have an API, it's just been a bit lower on my priority list!
I will make a couple changes which should improve things though in the next release.