I've never shot a WSM, but I have owned 3 standard length 7mm Mags. Using factory ammo, the recoil will be substantially more in a smaller, lighter rifle. I also haven't seen any data for the SM pushing a 150 Nosler much faster than 3050. My current 7mm RM pushes a 150 to 3200 over my chrono.
Marketing - pure and simple.