The opening chapters of Asimov's Foundation characterize Trantor as being a dense, highly-populated city, to the point that it's almost exclusively indoors and extends a mile underground.
But in the Encyclopedia Galactica entry on Trantor, Asimov states that Trantor has a land surface area of 75,000,000 square miles and a population over 40 billion. That works out to a population density of 534 people per square mile, no more dense than a small town.
Is there anything to account for the metropolitan, honeycombed structure of Trantor even though it has such a small population density?
EDIT: This question has a comment noting the population density and Asimov's later revision to Trantor's numbers, but it also states that it doesn't fully make up for the discrepancies.