I know about the fluting, I was just paraphrasing one of my books in that enlarging the caliber (in the case of the .40 that you mentioned) also reduced the weight without changing the size of the 1851, by virtue of more mat'l removed. I hope I said that better this time ??? Sometimes I don't express things well and was trying to contribute a bit to your post.
Before the spring steel came about, Colt was trying to explore all possibilities in manufacturing a larger caliber, lighter weight revolver on the '51 platform, as you stated, .40 was as large and as lightweight as he could get with the '51 frame, restricted at the time by metalurgy. Retaining the .36 caliber chambering was the best he could get with that platform.