Originally Posted by bocastephen

The goal of the winglet is basically nothing more than reduce induced drag - which occurs when the air flowing across the top of the wing curls downwards at the wingtip and wraps around the air flowing across the bottom of the wing which wraps upwards - creating the horizontal tornado flows called wingtip vortices....which at slow speeds and low altitudes create the wake turbulence that can cause accidents with smaller aircraft

The winglets interfere with this air flow phenomenon. As far as big versus small - I think it's just a design feature that takes into account the balance between the amount of induced drag created by that particular airfoil versus the trade off of additional parasite and form drag caused by the winglet itself.

The small winglets were earlier iterations of this concept - the A300 has tiny winglets, while the A319/320 has slightly larger versions of the same shaped winglet.

These new generation blended winglets might also provide additional lift while reducing drag, making them more efficient than the block-style winglets that just reduce induced drag. That is my best guess - I dont know enough about this new product to say what the specific technical advantage is.