FWIW, lots of HVAC guys don't do a proper job of sizing a heating plant...it is still very common for them to install one way too big for the actual needs. The thought is...they don't get called back if it's too big, but if it's too small, and can't keep the place warm, they will. But, too big means extra costs up front, poorer efficiency, shorter cycle times (lots more on/off cycles), and thus, often, shorter life and costs every billing cycle for fuel.
Under an ideal situation (impossible to produce), the heating system on the coldest design day, would run 100% of the time and just be able to keep up. Any time things are warmer, the heating system is too big, and needs to cycle on and off to keep from overheating the place. Cycles also mean less even temperature and wear and tear. A heating system becomes very inefficient if it doesn't run long enough to become fully warmed up internally, and then when it is shut off, that heat held internally is just, for the most part, wasted.