Bell wrote that he used lighter recoiling rifles because the heavier recoiling rounds were unsuitable when he perched above the tall grass on his tripod or ladders. He had heavier guns at hand when using the lighter pieces.
He did write something like that, which was referring to shooting over tall grasses while standing on his telescope tripod, or sitting on the shoulders of one of his men, but it was part of a passage listing all the different reasons he didn't like big bores.
As a rule he didnt have heavier rifles available and didn't feel he needed them, writing chapters of his books and articles explaining why.
He did own a 450/400 but never used it after the first safari he took it on and describes this in a letter to a friend. He purchased a pair of 416's from Rigby after they came out, but I cannot find anything about him using them or taking them to Africa. He mentioned a .350 Rigby Magnum, but it was about giving up on it because it was so heavy and didnt kill anything any quicker than his 7x57.
Bell plainly worked quite well with his light rifles (800 elephants with a 7x57 says it all) but he was also a gun nut who liked trying the latest thing.
He was a neophile. When the .22 Hipower came out he had to shoot an obstinacy of buffalo with one. When the 416 came out he bought two of them just to see what it was about. The .350 Rigby Mgagnum, same. When the Colt 1911 came out he bought one. When the .220 Swift came out he had to use it on red deer.
(I have references to him fitting some kind of a machine gun to his plane early in WW1, that was designed by someone he knew.)
Overall, although he doesn't say so explicitly, I think Bell thought of himself as a progressive man, and that he regarded big bores as old fashioned.
