I've never met Mr. Eastwood but your observations are likely correct.

I'm a six footer with big paws, a Mannlicher Schoenauer M1910 Takedown, and a .44Mag Ruger Vaquero.

I installed large (checkered walnut) custom wraparound grips on the Ruger so it wouldn't 'climb', and the MS does have a very narrow wrist. It's actually very svelte, throughout, as can be seen in the photo of Mr. Huston and the 'belly up' Mannlichers.

At the Mannlicher's widest point it is still rather an easy reach to press the magazine release with the left middle finger while balancing the rifle in the left palm. The right hand, with practice, catches the expelled rounds.