Skilled engravers were expensive and each sheet printed had to be hand-colored. He couldn't afford to worry about the artistic effect. His paintings excited European viewers, like no other, because Audubon also offered . . . Audubon started with the goal of producing prints of life-sized birds. The size gave him much more artistic freedom.
Some birds are tough to fit anyway.
Audubon could pose the birds in this manner because of his other innovation. Whereas Wilson always worked from stiff mounted birds, Audubon always used fresh-killed specimens.