Simply put, where the rod tip stops at the end of the back cast and forward cast will determine the size of the casting stroke. Generally, if the rod tip stays on a straight line during the cast and stops just below the fly line, the result will be a tight loop. Same for the back cast. If the rod tip tracks a convex or bicycle wheel shape the loops will be wide. Think of your rod tip as an anchor point for the fly line – where the rod tip stops will determine the shape of your casting loop. –Jim Solomon
The Fly Zone 