At the height of their powers, Grindelwald commanded the entire European continent save Britain, whereas Voldemort affected mostly just Britain. Since the population of the whole European continent is higher than Britain's, that means Grindelwald likely had more followers than Voldemort.
Why is Grindelwald considered second darkest wizard and not Voldemort?
There are several points to consider here:
Our main source for this claim is a book written by Rita Skeeter, who is not known for her fair, neutral, or indeed truthful reporting. Her assessment of which wizards or witches are the most dangerous cannot be taken as objective.
Also, Skeeter refers to Grindelwald only as not being the "Most Dangerous," by implication:
The name of Grindelwald is justly famous: In a list of Most Dangerous Dark Wizards of All Time, he would miss out on the top spot only because You-Know-Who arrived, a generation later, to steal his crown. As Grindelwald never extended his campaign of terror to Britain, however, the details of his rise to power are not widely known here.
Harry Potter and the Deathly Hallows
It is likely that Voldemort was indeed more dangerous or more powerful than Grindelwald, in terms of his skill at magic, without necessarily having more sway than he did, or more territory, or more followers. Remember, too, that Grindelwald’s power came in large part from the Elder Wand. While this was not known to most people, it was known to Dumbledore, who might have played a large part in shaping public perception of Grindelwald in Britain.
There could be an element of provincialism at play here. Recall that the previous quote indicates that Grindelwald never “extended his reign of terror to Britain,” and so many witches and wizards in Britain may be ignorant of how much power he wielded in the remainder of Europe.
Grindelwald may well have been less dangerous to the average wizard or witch than Voldemort. He certainly was less evil. Voldemort indiscriminately killed anyone in his way, whereas Grindelwald seems to have (often) imprisoned them. Voldemort was more of a psychopath than Grindelwald, more interested in overturning the established magical order (but less interested in revealing the magical world to Muggles).
