On one RS tour, I think it was in Penzance, our hotel was up a very steep hill. Luckily, the hotel took pity on us and sent a vehicle down to collect our luggage and bring it up to the hotel. We still had to convey ourselves.
On the same tour, one person had a large, serious backpacking type backpack. In a few days, she started wearing a second, similar one on her front. As the tour progressed, that second backpack got thicket and thicker. She bought more souvenirs than anyone else so by the end of the tour it was bigger than the one carrying her clothes.
And she was an assistant guide. I guess she didn't read Rick's suggestion on packing light.
So, while they recommend a carry on size bag for convenience and ease, you can bring whatever you want as long as you can handle it by yourself.