The Greek word translated as "thief" is lestes, which means "bandit," "robber." "brigand." It does not refer to simple theft per se in the sense of simply taking something that doesn't belong to you (that would be kleptos), but basically to gangs, which included highway robbers, but also outlaw insurgent groups. So they wouldn't have been crucified for what they stole (which typically would be money and food) but for being public menaces and/or perceived as insurgents. Basically they were seen as terrorists (and sometime they were terrorists).

It was more about social status than what the particular crime was. A major perk of citizenship was that citizens were (almost always) exempt from capital punishment and were instead fined or, in the case of high-profile crimes, exiled.

There is a possibility that the thieves were highway-men/bandits like those in the "Good Samaritan" story. They could have potentially been some sort of insurrectionists, adhering to anti-Roman zealotry, but that they were more like brigands is likely. The Greek word Saint Luke uses is kakourgos, the same noun in the "Good Samaritan" account. The other Gospels use lestes, a word with similar definition and connotation.

So, think more along the lines of armed robbery or political insurrectionist rather than someone who stole a loaf of bread in the market.