Is not this the fast that I choose: to loose the bonds of wickedness, to undo the thongs of the yoke, to let the oppressed go free, and to break every yoke?
One of the funniest things about the Sacrament of Reconciliation is the strange paroxysms that some opponents of the Church put themselves through to complain about penance. On the one hand, we're told that penance is an outrageous form of "works salvation" which allegedly requires us to do something in order to "earn" the grace of God. On the other hand, we're also told that penances are trivial and light and no work at all and that if the Church were serious, its priests would assign "meaningful" penances. So there you are: penance is a burden and it's not burden enough, according to the critics.
In reality, however, the Sacrament of Reconciliation is, like all things Catholic, incarnational. Repentance is supposed to issue in action, not just words. So the Church calls for some small act of penance to be done, not as a way of earning grace, but as a way of living out the grace we have received. Typically, this act is small precisely in order that it may not be a burden. But it is real because grace must be incarnate just as the Word had to become flesh. Real penance is not just a ritual: rather, it is a way of saying thanks!