Gellert Grindelwald has not been given enough importance in the series as his criminal activities were mostly in the past as compared to the timeline of Harry Potter and also not in Britain where most of the story was. So comparing his dark reign with that of Voldemort is a difficult task which deviates mostly from canon and makes way for certain assumptions, but I will try to do the job from the information we have from canon.
Grindelwald is a highly skilled wizard, compared to Dumbledore as an equal at the age of 17 and is known to be expelled from Durmstrangs for experiments he conducted,
Educated at Durmstrang, a school famous even then for its unfortunate tolerance of the Dark Arts, Grindelwald showed himself quite as precociously brilliant as Dumbledore.
...
At sixteen years old, even Durmstrang felt it could no longer turn a blind eye to the twisted experiments of Gellert Grindelwald, and he was expelled.
-Harry Potter and the Deathly Hallow's, Chapter 18:The Life and Lies of Albus Dumbledore
Grindelwald had plans to establish Wizard ruling on Muggles even from his school and he shares his ideas with Dumbledore when they meet each other at Godric's Hollow.
Gellert ---
Your point about Wizard dominance being FOR THE MUGGLES’ OWN GOOD --- this, I think, is the crucial point. Yes, we have been given power and yes, that power gives us the right to rule, but it also gives us responsibilities over the ruled. We must stress this point, it will be the foundation stone upon which we build. Where we are opposed, as we surely will be, this must be the basis of all our counterarguments. We seize control FOR THE GREATER GOOD. And from this it follows that where we meet resistance, we must use only the force that is necessary and no more. (This was your mistake at Durmstrang! But I do not complain, because if you had not been expelled, we would never have met.)
Albus
-Harry Potter and the Deathly Hallow's, Chapter 18:The Life and Lies of Albus Dumbledore
The ruling of Grindelwald had shown signs of Dumbledore's suggestions being implemented like having the slogan FOR THE GREATER GOOD and carving it on his entrance to prison Nurmengard, which he had build to hold his opponents.
“I[Hermione] think that's the worst bit. I know Bathilda thought it was all just talk, but 'For the Greater Good' became Grindelwald's slogan, his justification for all the atrocities he committed later. And . . . from that . . . it looks like Dumbledore gave him the idea. They say 'For the Greater Good' was even carved over the entrance to Nurmengard.”
-Harry Potter and the Deathly Hallow's, Chapter 18:The Life and Lies of Albus Dumbledore
Grindelwald's ruling was primarily dictatorship, he wanted to make Muggles believe that Wizard ruling will earn fruitful results and wanted to discard Statute of Secrecy, he was always compared with Adolf Hitler and alexwlchan's answer here draws an excellent similarities between them. Also this essay here.
Compared to Voldemort, he had enough kindness to build a prison for his opponents rather than just killing them with the Death Curse and he is not too kind to have morals for not killing them,
Krum’s jaw muscles worked as if he were chewing, then he said,
Grindelvald killed many people, my grandfather, for instance. Of course, he vos never powerful in this country, they said he feared Dumbledore
-Harry Potter and the Deathly Hallow's, Chapter 8:The Wedding
It's not new for a Dark Wizard to be fearful of Dumbledore, but he is the first of his kind.
Even though JK Rowling commented that Wizard wars and Muggle wars being interconnected, throwing all the deaths of World War II into Grindelwald's account is absurd.
Voldemort's killings will outnumber that of Grindelwald's because of his vast number of Muggle killings for Inferi, which Grindelwald had no mention about in canon. The number of people involved in Inferi are estimated by Slytherincess here.
So, through canon it is difficult to estimate the amount of killings Grindelwald committed because he is detached with the Britain community because of his fear in Dumbledore.