Tokyo is huge. It isn’t really one city, it’s more like twenty that all just happen to be next to each other, and that’s just the city proper. The Tokyo Metropolitan Area, also known as Greater Tokyo, includes eight prefectures: Tokyo, Chiba, Tochigi, Ibaraki, Saitama, Gunma, Kanagawa and Yamanashi. The total population of the region was estimated to be 35,676,000 people by the UN in 2007.
Using an approximate estimate of 36 million, this means Greater Tokyo is more populous than every country highlighted in red on the map below (click to enlarge):
To give you an idea of the area’s size, this is the Tokyo Metropolitan Area if it suddenly fell into the middle of Britain, killing millions of innocent people but creating a mildly interesting map: