This question is one of basic physics; less contact with the floor means less drag making a barrel turn easier. An empty wine barrel can weigh 140 lbs. and when wine is added that weight is greatly multiplied. The bulge in the middle makes corners easier to negotiate. Another advantage to the bulged shape is barrels will center themselves when laid in a cradle without shifting forward and back. The low center of gravity at the bulge anchors them in position.