It's all well and good to speculate but the assertion that is commonly made, as in the Uberti web site video and on the wikipedia page for the Walker, is that Samuel Walker specifically made a requirement to Samuel Colt that the Walker be capable of shooting horses.
So far, I've not been able to substantiate that claim.
I still very much believe that people have misunderstood the term "horse pistol" to mean it was for shooting horses. In fact, it was to be carried and used on horse. Other kinds of pistols are referred to as "belt pistols" and clearly these are for being carried on the belt not for shooting belts.
Others were known as "pocket pistols" and these were for being carried in pockets not for shooting them.
I have not yet encountered any real specifics from Walker concerning the design of the Walker revolver, other than some ergonomic issues concerning the sights and the grip.
It is my belief currently that Colt wanted to make as large a revolver as practically possible that could perform nearly as well as a rifle or carbine and have multi-shot capability.
An officer commenting on the Walker said it was as good as a rifle and better than a musket at 100 yards. At 60 grains of powder, it had the same service charge as later .58 rifled muskets would use. I think this is what Colt was trying to accomplish.
Steve