Sauron was 'greater', effectively, in the Second Age than Morgoth at the end of the First. Why? Because, though he was far smaller by natural stature, he had not yet fallen so low. Eventually he also squandered his power (of being) in the endeavour to gain control of others. But he was not obliged to expend so much of himself. To gain domination over Arda, Morgoth had let most of his being pass into the physical constituents of the Earth – hence all things that were born on Earth and lived on and by it, beasts or plants or incarnate spirits, were liable to be 'stained'. Morgoth at the time of the War of the Jewels had become permanently 'incarnate': for this reason he was afraid, and waged the war almost entirely by means of devices, or of subordinates and dominated creatures.
Sauron, however, inherited the 'corruption' of Arda, and only spent his (much more limited) power on the Rings...