The terms "bull barrel" and "bull gun" do not come from any comparison to the size and weight of the male bovine, but from Freeman Bull, a Springfield Armory employee who advocated using heavy barrels for accuracy in the Armory's match rifles.
I was really, really, really tempted to call BS on this, but here's more from a document on the National Park Service web site:
Freeman R. Bull &
The Springfield Armory Rifle Team
Freeman R. Bull deserves a large portion of the credit for introducing the shooting world to the precision of the Springfield rifle. Bull, machinist and gauge maker at Springfield Armory, began shooting Springfield rifles in competitions as early as 1875. He and his fellow Springfield Armory Rifle Club members profoundly influenced the design and manufacture of all small arms.
Bull built precision adjustable sights for Springfield match rifles. Before computer advanced ballistics, he developed accurate sights by firing the arms themselves. His sight designs were later adopted for use on U.S. service rifles. One of these was the Model 1884 Buffington Sight used on the later Krag*Jorgensen Rifles and the Model 1903s.
Bull's legacy as a sportsman continues today with the application of the term "bull" to describe the extra-heavy match barrels used in competitions. Freeman R. Bull was one of the first to fit such barrels to military rifles.