EMI is complex.
If I were to boil it down to the simplest idea I can think of... well that would be how well matched is the impedance of the circuit that will radiate to the electromagnetic (EM) impedance of free space. When the two sides are matched up fairly well then the power flows right out as EMI, but if there is a big difference then the power is reflected.
I am guessing that was no help, and I may even be confused about this stuff. The real answer is to study EM until it starts to make sense (and the math to help it make sense). I am not saying you need to spend monies to study by the way.