I always thought that a comment would be, in comparison to other kind of response, such as an answer, something brief. However, it doesn't seems like Wiktionary says anything about it. Am I mistaken?
The nature of a comment, when the meaning as "a remark expressing anopinion or reaction," is to be brief. That's why Stackexchange sites, for example, allow you a mere 600 characters to write one.
A commentary can be longer, even the length of a bible. Comment can carry the meaning of commentary, but that is an archaic usage.