If it's supposed to be a bowlance and it's a metal casting, correction is easy. By lessening the curvature of the bow, you would likely get a properly scaled bowlance.
According to the Osprey books, Tribes of the Sioux Nation, by Michael Johnson and Jonathan Smith, and Warriors at the Little Bighorn, and American Plains Indians, by Richard and Jason Hook the bowlance was used by the Cheyenne and Sioux as late as the 1880's.
Wooden Leg (17 to 19 at the time) was at the Little Bighorn and Amos Bad Heart Bull is credited with a pictograph from the depicting the carrying of a bowlance at Greasy Grass.
Perhaps the painter misinterpreted this ceremonial weapon and "corrected" it.