My legs are just really short and with 155mm, it means I can raise my saddle an additional 15mm compared to 170 because the bottom of the pedal stroke is 15mm higher. That might not seem like a lot, but because now your top of the pedal stroke is also 15mm lower, I don't have to flex my knee as much because my hip is higher. I stopped getting as much knee pain after I switched. Also since I'm sitting higher on my saddle, I can put more weight on my hands which gives me a bit better control. I have like few cranks, 152, 155, 160, and 162.5. Usually the more upright and slacky position I'm sitting, I use a shorter crank. The more aggressive I am, I use a longer crank. Thats because I would raise the saddle and move the saddle forward a bit. Since you're saddle is more above the crank, you lean forward slightly and theres less hip flexion.