I’m not sure I get your issue. If I right-click on the thumbnail from your oneboxes to open it in a new tab, I get a fairly small picture, both in dimensions and size:
The original image (<meta data-rh="true" property="og:image" content="https://storage.googleapis.com/afs-prod/media/f07fb1af3b834c88b858171d2ed8d594/3000.webp">, however, is quite large indeed:
https://storage.googleapis.com/afs-prod/media/f07fb1af3b834c88b858171d2ed8d594/3000.webp
So, it seems properly resized and compressed by Discourse when it downloads the remote image.
Be aware that, as far as I know, there can be a wait of a few minutes before the image is downloaded by Discourse.