In a way, you could make an argument that prison should not have timed sentences, but rather that it should be "you stay until it is deemed that you will not offend again, and that you can be a productive member of society."
After all the word "corrections" implies that something should be corrected, not that somebody should be punished.
If it were only for corrections, then it would cause all kinds of problems. The most obvious one is that you get one "free crime" to commit.
There's also a fairness (and rule-of-law) argument: if two people steal $100 each, why would one person be let out earlier?
If that's your justification then the horse has already left the barn, developed space travel and colonized Mars. Look at the number of factors that go into sentencing -- or just the definition of the crime, like what was stolen (distinct from its market value) and how (e.g. on a computer vs. in person vs. in a conspiracy with accomplices), then there is a lot of flexibility for the sentence itself (most sentences are "up to" X penalty) with a whole list of mitigating and exacerbating circumstances in the sentencing guidelines, subsequent parole for good behavior in prison, etc. etc.
Even if prison was entirely intended for corrections, it is punishment (a denial of freedom). So there is a fairness argument that if two people do reasonably similar crimes that they should get comparable sentences. That is just one argument though, and fairness (as always) is weighed against other arguments.