Lanchester's square law.
Basically assigns a 5 man squad a value of 25 vs a 4 man squads 16. The 5 man squad is able to reduce the firepower of the 4 man squad ( by killing a model) early, then having a 5 - 3 ( 25:9) advantage. If the 5 man squad is able to kill another man before losing a model then it's advantage only escalates.
Quantity has a quality all of it's own is mathematically true, which is why you are often better back teching to less powerful units in order to have a numerical parity or superiority.
There are other factors at play, combat persistence in particular, however some of the higher level functions such as ammunition do not apply in COH2.
Another related concept is volleys. Currently I worked out that one Volks model will be dropped by 5 volleys from IS on average ( assuming they all target the same model). If however you use veterancy, equipment ( Brens) or bulletins combined to reduce this figure by a full volley then your infantry will have a decisive advantage.
Kind of curious what this looks like if you test it in game (e.g. 50 engagements between IS and Grens/Volks and see how many more you win with the 5th man)