Rabbi Johanan b. Zakkai explained why a thief is punished more than a robber: the latter considered God and a slave alike. The former treated God even less, as one who could neither see nor hear. Rabbi Meir in the name of Rabbi Gamliel said that the one was like a man inviting all the people except the prince, the other like one not inviting anyone.
Why a Thief Is Punished More Than a Robber in Jewish Law
Gaster, The Exempla of the Rabbis (1924), no. 278