פירוש על קידושין 100:3

In the cases in the first clause, I might have thought that the gifts were not kiddushin because the man might not have known that he had given less than a perutah for each woman he was betrothing. After all, it is hard to distinguish between a perutah and less than a perutah. He might have thought that he performed kiddushin, and that the gifts were just gifts.
But when it comes to the minor who betroths, we might have thought that everyone knows that minors cannot betroth. Therefore, when he sends gifts as an adult, this is kiddushin. Therefore, the mishnah had to teach us that also in this case the gifts do not count as kiddushin.