Physics,
Big guns kick, and to be managable need to have some weight to them. Hence there is no benefit in going to a shorter lighter action. So if you don't need the shorter lighter action, why would you give up magazine capacity and case capacity? Most of the big bores need all the powder capacity they can get.
And my idea of thumpers start in the 40 cals pushing 400's 2400 fps, under 40 is medium bore.
