As an example we will look at HAT WWI sets.
German Inf- 25mm
Heavy weapons- 23mm
Ruga Ruga- 24.5mm
If one wants 'real life' as a reason then it would appear the heavy weapons would need 'bigger' guys to carry the load.
The 'light' units a bit smaller. African troops in 'uniform' should not be smaller then the 'native' units......