What do we learn from "Me'asef l'Chol ha'Machanos"?
Rashi (citing the Yerushalmi in Eruvin, 5:1): Since Dan was [the most] populous 1 , they traveled in the rear, and whoever dropped something, they would return it to him.
Rashi #2: This teaches that the Shevatim traveled (one behind the other) like a beam. 2 (Dan was last.)
Rashi #3 (in Yeshayah, 2:12), Da'as Zekenim, Hadar Zekenim (2): Those who travel at the back are called 'Me'asef' because they pick up all the stragglers.
Rashi writes that Dan was [the most] populous. Yehudah was bigger!