I'm going to suggest modifying this statement a little. It's not that high aspect ratio foils produce more lift (for a given amount of area), they create less induced drag. Here's a bit from North Sails:High aspect foils develop more lift for their size than low aspect foils.
This article was using an airplane wing as an example, so for a given wing area, increasing the span increases the aspect ratio.Another factor that must be considered is induced drag. This is the drag that a wing generates when it creates lift. Over most of a wing, the low pressure above the wing is kept isolated from the higher pressure underneath by the physical presence of the wing. At the tip of a wing, where the wing ends, there is nothing preventing air from flowing around the wing tip from the high pressure beneath to the lower pressure above. This results in the standard tip vortex that is often seen spinning off the tips of airplane wings and flaps. When the flow takes this alternate path around the tip instead of over the airfoil surface, energy is expended that does not develop lift, but does cause drag. This is called induced drag and it increases exponentially with lift, so a wing, such as a sail, that is producing substantial lift, experiences much more induced drag than a wing that is producing a lesser amount of lift.
The most effective way to minimize induced drag is to increase span, as induced drag is inversely proportional to the span squared. Highly efficient airplanes like gliders have very high span for the amount of lift they are producing. Winglets are a way to create the effect of higher span without actually increasing the physical span. They are useful when there is an artificial constraint on wingspan (like a draft limitation on a keel).