Objection 1. It would seem that theft is a more grievous sin than robbery. For theft adds fraud and guile to the taking of another's property: and these things are not found in robbery. Now fraud and guile are sinful in themselves, as stated above (55, 4,5). Therefore theft is a more grievous sin than robbery.
Objection 2. Further, shame is fear about a wicked deed, as stated in Ethic. iv, 9. Now men are more ashamed of theft than of robbery. Therefore theft is more wicked than robbery.
Objection 3. Further, the more persons a sin injures the more grievous it would seem to be. Now the great and the lowly may be injured by theft: whereas only the weak can be injured by robbery, since it is possible to use violence towards them. Therefore the sin of theft seems to be more grievous than the sin of robbery.
On the contrary, According to the laws robbery is more severely punished than theft.
I answer that, Robbery and theft are sinful, as stated above (A4,6), on account of the involuntariness on the part of the person from whom something is taken: yet so that in theft the involuntariness is due to ignorance, whereas in robbery it is due to violence. Now a thing is more involuntary through violence than through ignorance, because violence is more directly opposed to the will than ignorance. Therefore robbery is a more grievous sin than theft. There is also another reason, since robbery not only inflicts a loss on a person in his things, but also conduces to the ignominy and injury of his person, and this is of graver import than fraud or guile which belong to theft.
Hence the Reply to the First Objection is evident.
Reply to Objection 2. Men who adhere to sensible things think more of external strength which is evidenced in robbery, than of internal virtue which is forfeit through sin: wherefore they are less ashamed of robbery than of theft.
Reply to Objection 3. Although more persons may be injured by theft than by robbery, yet more grievous injuries may be inflicted by robbery than by theft: for which reason also robbery is more odious.
Source: Summa Theologica (New Advent)