I mean, you need to account for how Alexander got to the Indus from Macedonia all in one burst of conquest, whereas Chandragupta Maurya simply ventured from Bihar to Bactria and the Deccan, a far smaller distance. And Alexander conquered the entirety Persian Empire, the largest empire in history until that point, whereas Chandragupta Maurya’s greatest conquest was “merely” the Nanda Empire. So, looking at Alexander’s conquest of the Paurava kingdom and Chandragupta Maurya’s conquest of Bactria in isolation, Chandragupta Maurya was greater, but in context, the fact that Alexander even got to Punjab is a wonder unto itself.
That doesn’t stop the fact that the Indus was the end of the supply line of any “Persian” empire, especially one with a capital in Babylon, and Chandragupta Maurya would be a much greater general than anyone Alexander fought.