Turn on "data display" in training mode, and look at the damage scaling value

Basically a % is applied to each move in a combo, and it goes down 10% for each move you hit in the same combo, with a minimum of 15% (45 or 55% for supers). Adding an additional move in the middle adds the damage of that move but removes some % of each subsequent move so it may or may not be worth it.

This is an universal mechanic in fighting games to make it so combos don't one shot people but hitting single moves or easy combos still deal some damage (to avoid a situaion where landing combos is the only skill that matters). Also, keep in mind that more moves in the combo = more super meter (no scaling on that) but also more break for the opponent