Two different jobs, two different numbers
A combi has to do two things. Heat your radiators, and make hot water on demand. Those need very different amounts of power, and it's nearly always the hot water that sets the size.
Heating the radiators in a typical three-bed semi takes surprisingly little, often under 10kW. Producing a decent shower flow instantly takes a great deal more. So when you see a 30kW combi going into a house with a 9kW heating load, that isn't a mistake. The extra output is there for the shower.
What decides it
- Number of radiators and the size of the property, which sets the heating load
- Number of bathrooms and how they're used, which usually sets hot water demand and therefore the size
- Incoming mains flow rate, because there's no sense specifying a boiler that can heat more water per minute than your mains can deliver
- Insulation and glazing, since a well-insulated house needs less
- Whether you're on a system boiler with a cylinder, where the sizing logic changes completely
Why oversizing costs real money
A boiler running well below its minimum output can't modulate down far enough. So it fires, meets the demand, shuts off, cools, and fires again. That cycling is the biggest single cause of early component failure in domestic boilers, and it means the efficiency figure on the box bears very little relation to what you actually get.
The right size is the smallest one that comfortably meets your real demand. Getting there takes five questions, not a guess.