SND
The fact that the aircraft is designed around certain weights.
As you know, aircraft G forces change in flight, therefore aircraft weight changes in flight too (not mass). So to answer my earlier question "What's the difference between taking off at max weight and climbing out at max power, and taking off over weight and climbing out at max power?"
If you are climbing out at MCP you will be the same load/stress at what ever weight, right? Lighter = higher ROC, heavier = lower ROC. Too heavy = no ROC.