Forgot about this with all the holiday stuff.
I will make a simple statement that the same photos on Yahoo were displayed without any issues. It is not the photos. To be precise, I did not transfer photos from Yahoo. I uploaded the same image files from my computer to Yahoo.
And as far as upsizing, I checked again, and the image displayed when selected does not fill up the entire screen leaving white space and part of the breadcrumb visible. The downloaded image (1280 x1278) when shown on my desktop is slightly larger filling the screen space top to bottom with no white space. If I upload the photo at 720x719 size, the selected image is the same size as the 1280x1278, then the downloaded image is smaller.
Don't know if any of this helps, but the images on Groups.io are blurry, and there is nothing I can do about it.