It's tempting to assume a bigger AC unit always cools better, but sizing an AC system is about matching capacity to the space, not maximizing it.
An oversized unit cools a room quickly but shuts off before it has run long enough to properly remove humidity from the air, a pattern known as short-cycling. The room may hit its target temperature but still feel damp and uncomfortable, and the frequent on-off cycling adds wear to the system.
An undersized unit has the opposite problem: it runs nearly nonstop trying to keep up, straining its components and often still failing to reach a comfortable temperature during the hottest parts of the day.
Proper sizing accounts for room dimensions, ceiling height, window size and orientation, insulation, and how many people typically use the space — which is why a walkthrough and load calculation matters more than picking a unit based on room square footage alone.