In order to answer this question one has to evaluate what exactly we are addressing. If we are talking about the ethics behind eating animals, then deontologically speaking one would consume both animals. Simply because there is no reason why the life of one individual is more valuable than that of another. One thing to note is that value isn't always interchangeable with usefulness, i.e some people may point out that dogs function better as domestic animals who have aided humans for centuries, in such a case one could apply the same rule to suggest that a rich business is more valuable because he contributes more to the economy than someone who is unemployed. Note that while the businessmen might be more valuable to society or the economy, his life is no more valuable than someone else's. However humans do not operate on fixed notions, rather the majority are known as "rational particularists". What this means is that, in being rational, they will take each situation selectively and not base it on rules. So for example if there was a talented pig who could talk, a human would value that pig higher than any dog. Now what happens with particularism is that people create general trend lines. As in if I were to equate the average dog, with the average pig, who is more valuable. And for most people who have lived with dogs, it seems rational to assume that dogs are valuable for the assets they provide to humans such as but not limited to, "making us happy, helping the police force, etc". As humans cannot find a relative use for pigs other than to serve as their dinner, it seems rational to draw a general trend that the life of a dog is more beneficial to humans than the life of a pig. Biologically speaking a pig only takes 4 months to mature while a dog takes up to 2 years. Therefore to meet consumer production it is easier to house pigs which die faster than dogs which take longer to raise. There is also a cultural element for example in India it is the custom for hindu's to worship cows as they were known as the milk givers, However in the west we have little relationship with cows because we take their anatomy for granted. This then suggests that culture creates tendencies to favour one animal over another even if the reasoning doesn't follow the same logic. I know I haven't really answered your question as to why pigs are less valuable but I hope that i provided insight.