When you multiply something by a hundred, you get more or bigger, right? In math class we used to add two zeros to a number to multiply it by a hundred. Well, take a look at this sentence:
The galaxies – 100 times less massive than our Milky Way – are now among the smallest known to host such big black holes.
Can you have something a hundred times less? If you can, what are you multiplying by a hundred?
Yes, the sentence is idiomatic, but still, I think it’s better to write what you mean. The sentence means one hundredth as massive.
Oh—here’s a picture: