I think a key part is they need less fuel since they aren't going all the way to orbit. Getting to orbit is the difficult part and it's why they need the whole

I think a key part is they need less fuel since they aren't going all the way to orbit. Getting to orbit is the difficult part and it's why they need the whole gigantic first stage booster. When they first developed the second stage they did some low altitude test flights with just the second stage and it went miles into the air IIRC, albeit with no payload. It's possible that once they deliver the payload, the second stage might have enough fuel to fly back to some drone ship or something.