Several hive-cities grouped closely together can merge, forming continent-spanning "hive clusters", also known as "mega-cities" outside the Imperium. In extreme cases, hive clusters/mega-cities can cover much of an entire planet's surface, forming a near-uninterrupted ecumenopolis.
A hive-city typically houses between 10 million to several billion residents, who dwell in massive self-contained habitation blocks. Designed to house a rapidly growing population efficiently, a single hab-block may house up to a million residents. The emergence of the first mile-high self-contained hab blocks in urban areas is a telltale sign of an industrialized secondary world growing to become a primary world, hive cities being characteristic to primary worlds of major economic and political importance. Overtime, as hab-blocks agglomerate into a single near-monolithic structure, they are simply built over, new and new layers of the ever-expanding city growing on top of the older strata, eventually forming a massive man-made mountain consisting of hundreds of interconnected spires and often towering well into the planetary stratosphere.
Hive cities are so massive that constructing (and demolishing) them requires a whole new take on engineering. One instrumental aspect of constructing such colossal structures is the use of macrofabrication techniques, which essentially entail the use of 3D printing on a massive scale. Demolishing or attacking a hive-city likewise requires engineering considerations normally reserved for natural formations like mountain ranges - indeed, even an average hive-city is large enough to form it's own weather system and significantly effect weather for hundreds of miles around. Simply put, the sheer bulk of hive cities allows them to weather even orbital bombardment with only superficial damage, and also means the structures are affected by natural phenomena like earthquakes more like solid mountains than cities.
Much of the material that hive cities are built up from is excavated from underneath and processed into building materials. Consequently, hive cities usually stretch at least as deep down as they are high, housing much of their actual industrial capacity and utilities deep underground, where the overlying mass of the city shields them from all but the most heavy bombardments. Overtime, some of these deep areas may flood or cave in from a lack of maintenance (since keeping track of every single hallway and tunnel in a structure of this size is extremely challenging to say the least), be contaminated by toxic leaks or otherwise rendered uninhabitable. These areas are sealed off, only the bravest and foolhardiest daring to venture in them, for all sorts of dangerous creatures and people tend to make home in the underhive.
Most of a hive-city's residents dwell within 500 levels of the planetary surface. The quarters are cramped, citizens often having to live in shared dormitories, but the Imperial authorities make sure to provide at least the basic necessities to all who reside here. Those who have distinguished themselves somewhat are assigned larger private apartments in higher levels. At the very top of the city, the Sidh society's elites live in luxurious penthouse apartments within the city's many spires.
Hive cities usually end rather crassly, having no sub-urban districts whatsoever, being surrounded by empty landscape. In many cases, this landscape is little more than a toxic wasteland choked with industrial pollution, roamed by outcasts and scavengers. In other places where the cities are carefully managed, this landscape might as well be a forest or vast corn fields, providing food for the city's residents.
Hive-worlders may live out their entire lives without ever seeing the sun of their homeworld or ever setting foot on its natural surface. Many consequently suffer greatly from agoraphobia when exposed to non-urbanized landscapes.
The massive populations of hive worlds mean extreme consumption of resources. In keeping with their frugal mindset, Sidhae especially on hive worlds let nothing go to waste, meticulously recycling everything that can possibly be recycled (including even the dead). Consequently, imported unrecycled raw materials often command premium prices - in extreme cases, a fresh piece of plastic can cost it's own weight in gold on a hive world, although such cases are exceptions rather than rule. However, any foe who might attempt to starve a hive world into submission simply by laying siege to it will learn otherwise quickly - in keeping with their autarkic, self-sufficient mindset, Sidh city-planners usually include sufficient facilities to synthesize nutrients for the population in.
The line between a primary world and a hive world is blurred somewhat, primary worlds usually having at least a couple hive-cities, but true hive-worlds usually having dozens of hives aglomerated into hive clusters. In keeping with the Sidh environmentalist principles, it's usually worlds with little to no native life that are designated to become hive worlds.