The wing configuration is an average design for optimization. Each configuration or use, would have a different optimized wing design.
The 737 first went into service in 1968 (?!?!?) damn!
Computer modelling and design have changed a bit, and due to access and CAT, the wingspan is stuck. turn em up, turn em down, or both..the longer and thinner at the end, the less 'theoretical' drag, and better economy, theoretically.
Of course, it has nothing to do with the ever reducing size or lack of outboard flaps...negating the size of that big round dork hanging off the bottom of the wing structure!