That seems to be based on the population of the city proper for both - Irbid definitely has a much larger population for its metropolitan area, whereas I can't find a separate metro population for Zarqa. Perhaps that's the source of the confusion?
Are you sure Villa Nueva is the second biggest city in Guatemala? I have been there many many times and I haven't even heard of it. Maybe it's on the coast? Weird.
Zarqa is part of Amman's metro area, to be precise. It's all one contiguous urban area. Same with Villa Nueva/Guatemala City. That's why it's callled 'New City.' :)
That seems to be based on the population of the city proper for both - Irbid definitely has a much larger population for its metropolitan area, whereas I can't find a separate metro population for Zarqa. Perhaps that's the source of the confusion?