I just dug a little into the Flutter API and saw what might be going on. Scaling is done properly on a regular browser, but there might be a built-in function for it somewhere. It may take a few more steps with Flutter.
(Just rambling a bit for those who care about details and theory crafting.)
Calling BoxFit in Flutter is significantly less expensive for thumbnails, I would speculate. It appears that it would take a few operations to scale an image properly and could impact slower devices.
An option to enable or disable image scaling would be nice. It’s just annoying that its probably not a one line code change. (Unless it is. That would be great.)