what you're calling a "threshold" most RTS players would probably call "critical mass", and it applies mainly to the situation where a ranged unit no longer needs a frontline, or where a group of backline units can shrug off most direct attacks, because nothing can tactically close the gap
it's largely a matter of targeting and accuracy since smaller groups absorb random non-splash shots more evenly and thus won't get enough shots off, but any unit with any combination of long range, a slow firing rate, high accuracy, passive self-maintenance, a force-multiplier and a splashy warhead is a unit that is more likely to cross that threshold