Leviticus 5 gives instructions for offering a lamb for a known sin of trespass, but a ram for unknowingly trespassing against the Lord. What is the difference in the types of sacrifice?
A classic explanation is, because when a man knows that he faulted, he would regret more than a man that just doubts. So in case of a known sin, the qorban is just a lamb, instead of a ram for a "not-sure" case.
In Guide for the Perplexed 3:26 Rambam writes:
You ask why must a lamb be sacrificed and not a ram? but the same question would be asked, why a ram had been commanded instead of a lamb, so long as one particular kind is required. The same is to be said as to the question why were seven lambs sacrificed and not eight; the same question might have been asked if there were eight, ten, or twenty lambs, so long as some definite number of lambs were sacrificed. It is almost similar to the nature of a thing which can receive different forms, but actually receives one of them. We must not ask why it has this form and not another which is likewise possible, because we should have to ask the same question if instead of its actual form the thing had any of the other possible forms. Note this, and understand it. (Friedlander translation)