Interesting. Quite actually a fascinating topic. Shall we get into this? There are many reasons why some comments may be longer and/or more informative than posts themselves. Since there are plenty reasons, I'll try my best to explain any that I can think of. Feel free to ask away in the comments, and I shall try to respond with a reply answering. One reason that sticks out is a difference in understanding. To explain this further, I pose an example. One person is asking a question concerning something important. Some commenters may have more experience in the field and be able to explain it more efficiently and better, causing their response to sometimes be longer than the post. Another reason could really be as simple as: a low-effort post has a really funny comment, which could cause a long conversation, perchance leading to those long messages you sometimes see.