Population: Midgar
Geographical size: Esthar
That's my guesstimate. Esthar, when looking at it, had a lot of tubes and so forth but everything seemed very spread out, similar to Auckland in New Zealand which is, I think, larger geographically than London despite London having about ten times the population. Midgar, however, has slums, upper and lower cities, etc. etc. which seemed to me to be massive. So yarrr. Zanarkand might have managed to have a higher population than Midgar before it was destroyed, but it's hard to say. It still came across as less densely populated than Midgar, although like Esthar it seemed to be bigger in geographical size (although not even close to as large as Esthar itself, which is insanely spread out).