Sometimes it's harder to replace more. Say both thieves had spent their spoils & are caught. It'd be easier to replace the $100 versus the $1,000. The act may be the same & the act may be carried out again by both parties, but in the end it's really 'bout payin' restitution to the courts of like 1,000%+.
This is similar to the attempted murder vs murder argument. Both criminals have the same mind-set here, and both need to be rehabilitated as such. If you're capable of stealing five dollars from a child or a million dollars from a bank, the fact is you are capable of stealing and that must be corrected. It's silly, but having different punishments for the same crime is like telling the criminal, "Don't feel bad about robbing the convenience store, the law says it's not nearly as bad as robbing the bank across the street!"
Although when you through the Robin Hood angle into the mix, it makes matters that much more confusing.