That's not true in all RTSes. Take StarCraft, for example, and there are plenty of games on record that were decided not just by 1 unit, but by 1 attack from 1 unit. There are Zerg players, for example, who have developed a reputation for creating havoc after getting a single zergling (the smallest and cheapest attacking unit) into their opponent's base. A single shot from a Protoss reaver can mean the difference between taking minimal damage and losing half of your workers (and subsequently the game).
But I'm thinking about TA-style games, the topic of this discussion, which pride themselves on large armies. Though, to your point, early game of Supreme Commander is also quite chess-like, because of how restricted the set of opportunities is.