How much heat you get from a radiator is ultimately determined by its overall surface area.
There are some radiators that have been cleverly designed to incorporate extra fins or convectors to help achieve a desired level of heat without taking up loads of wall space. This can often be the case in rooms, such as kitchens, where wall space can be at a premium or on short walls, such as beside a set of bi-fold doors.
Considering ‘double’ radiators, where there are two sets of bars or tubes, can also mean having from heat from a single radiator. It can also mean the difference between buying two radiators or just needing the one.