An RF-s 600mm f/4 wouldn't be any smaller as the front element would still have to be 150mm to get f/4 : that's what f/4 means f= 600mm / 4 = 150mm .Oi....that could fit on r/ConfidentlyIncorrect....
"Advanced Photo System (APS) is a discontinued film format for still photography first produced in 1996." *Wikipedia
It was the last new film format before digital became a common thing. Reason were in fact the ability to build smaller and cheaper cameras with smaller lenses.
If they'll do produce APS-C R cameras, APS-C Lenses would make a lot of sense for both advanced and budget users.
Budget users can get affordable and compact lenses at all, while birders (or who whatever) can get cheaper, lighter or longer telephoto lenses (choose 2),
just imagine super tele RF-S L lenses. maybe a RF-S 600mm f/4 L which is way lighter and not as expensive as its RF counterpart. or one with an bigger aperture while maintaining the same weight and price.....just imagine...
I mean - I am not into that super telephoto range, but if I were and I buy an professional APS-C camera to get more reach, I would too be very happy about lenses, especially for that. I mean with FF lenses on APS-C - you waste a lot of money and you carry a lot of weight for an image circle, you are actually not using at all.
And I don't see any typical rebel user jumping from rebel and efs to r9 or r8 with those very expensive RF lenses. They also need totally different focal lengths for standards - now there is only 2 24-105s (not enough wide angle) or the very very expensive 15-35L (not enough tele). For the start you can continue to use the EFs lenses, but they won't be available new for long.
The only part that could be made smaller are the rear elements which would make very little difference .
A freznell element DO version would be a lot shorter and a bit lighter but the front element would still be the same size and that would be much the same crop or full frame.
A bigger aperture 600mm such as f/2.8 would be colossal as 600 / 2.8 = 214mm and insanely expensive and ridiculously heavy whether it was crop or full frame.