Nick is shocked when he comes home at two o'clock in the morning because the "whole corner of the peninsula was blazing with light" from Gatsby's house. He was actually fearful for a moment that his own house might be on fire! Once he sees that all Gatsby's lights are on, he assumes it must be another raucous party, but it isn't. The place is empty except for the man himself: Gatsby. When Nick tells him that his "'place looks like the World's Fair," he seems to refer to both the size and the scope of the mansion and its lights. The light being produced is so prodigious because it is a huge house and every light seems to be on.
The comment seems to imply a bit of a censure as well: why on Earth does Gatsby need to have all these lights on? The answer is simply that he doesn't. Perhaps he wants to make sure Nick knows that he is awake so that they can discuss Nick's inviting Daisy to tea. Perhaps he is hoping Daisy might see his house across the way. Either way, it's kind of ridiculous and over-the-top, just as Gatsby himself can be. His car is over-the-top, as are his parties, his library, his "business," and his persona.