I recently travelled in a ATA 737-800 from Chicago Midway to DFW and back and was impressed by the efficiency of the airline. Flights departed and arrived in time. The staff were polite and helpful.
I was fortunate to sit on a window seat on both the flights. I observed that the winglets were rather large in proportion to the Wing. In comparison, the winglets on a 747-400 don't seem to be as large. Also it seems that the some body on the manufacturing floor used a heated rod to bend up the wings near the tip . The winglets raise sharply from the wing on other airplanes such as the 747-400 or MD-11.
What factors are taken into consideration when designing winglets ?