The Rambam, in his Moreh Nevuchim, explains that the relative severity of the sin of selling and slaughtering stolen livestock, as described in this Parsha (שמות כ"א:ל"ז), which requires a stiff penalty, is based on the fact that thieves customarily try to sell or slaughter a stolen animal in order to make their act untraceable. In just the same way, he explains the severity of stealing sheep and cattle over stealing other items. The more susceptible something is to being stolen, the more severe the punishment is for stealing it. Livestock is usually kept in the fields and cannot be guarded as one would guard a household object; the theft of livestock is therefore a more severe sin.
Another interpretation is suggested by the Abarbanel. He notes that the Torah does not say to pay "five male oxen per male ox and four male sheep per male sheep;" it rather speaks in more generic terms: five oxen per ox, and four sheep per sheep (שם). The general penalty for unconfessed theft is double-valued payments (שם כ"ב:ג'). Regarding sheep and cattle, however, once the animal is stolen or slaughtered, correct assessment becomes impossible, because the thief can claim that the animal was worth less than it actually was. Bearing in mind that male sheep and cattle are worth more than females, the Torah therefore reassessed the double-value payment in such cases. For stealing one male, the thief must pay five oxen of either gender, each of which may be worth less than the original stolen ox. Likewise, for stealing one sheep, he must pay at least the value of four female sheep. This assures that the victim will get paid no less than the amount of money he lost. The Abarbanel then adds in general that, "if this is the Rabbinic tradition regarding these laws, we must accept it with happiness."