Scientific classification, taxonomy and geographical categorization conventionally include the adjectives greater and lesser, when a large or small variety of an item is meant, as in the greater celandine as opposed to the lesser celandine. These adjectives may at first sight appear as a kind of null comparative, when as is usual, they are cited without their opposite counterpart. It is clear however, when reference literature is consulted that an entirely different variety of animal, scientific or geographical object is intended. Thus it may be found, for example, that the lesser panda entails a giant panda variety, and a gazetteer would establish that there are the Lesser Antilles as well as the Greater Antilles.
It is in the nature of grammatical conventions evolving over time that it is difficult to establish when they first became widely accepted, but both greater and lesser in these instances have over time become mere adjectives (or adverbial constructs), so losing their comparative connotation.
When referring to metropolitan areas, Greater indicates that adjacent areas such as suburbs are being included. Although it implies a comparison with a narrower definition that refers to a central city only, such as Greater London versus the City of London, or Greater New York versus New York City, it is not part of the "comparative" in the grammatical sense this article describes. A comparative always compares something directly with something else.