Why is the inductor value needed reducing as we increase the peak
Firstly, if you want to regulate voltage on a more powerful load, the peak current will clearly be higher. A higher peak load current necessitates a lower value of inductance because if the inductance is too high, its natural series resistance will be too high and losses will be greater (resistive and core losses) or the inductor will be bigger and more costlier.
On the other hand, you could decide to design your buck circuit to be able to deal with heavy loads but only run it on a light load so why bother with using a higher value inductor? If you are designing a buck circuit to run with a light load then it makes sense to choose a higher value of inductance because then, the losses (at low loads), will not be as much i.e. you design it to be as efficient as you can for the load range you want.