First, I want to note that I agree with almost everything in Glen's answer. But I think there's merit in focusing on a different aspect of your question.
You should, in general, refrain from answering questions in the comments. Comments are intended to request clarification or suggest corrections. They are not answers. Answers can be edited by other users, they can be individually responded to, and they hold up better over time.
If enough comments are made on a question in a short period of time, you comment-answer may almost immediately be hidden. There's no way for the community to "check" comments through voting. There are only two options, upvoting and flagging for deletion. You can't downvote a bad comment-answer, and asking the mods to determine if something is incorrect enough to be deleted is unfair and not how SE is supposed to work.
Answering a question with a one sentence answer usually is an indicator that either the answer should be expounded upon or the question needs to be improved. However, there are certainly going to be cases where an answer can completely and correctly address a good question with only a couple sentences. This doesn't mean that it shouldn't be posted as an answer, it just means that you answer will be short. But then your short answer can be reviewed and voted on by the community, it can be accepted by the asker, and it will be preserved for future people who have the same question.
Remember, while we certainly are aiming to answer individual people's questions as they come to us, one of the overall goals of each community is to create a lasting resource preserving the Wisdom of the Ancients.
As for whether or not you should go back and turn your comment-answers into full answers? I think that's up to you. If you can expand on your short answer, do so. If someone else wrote an answer that you can't add to or improve upon, then it's probably not necessary for that question. Doing it simply for the purpose of driving up our answer:question ratio isn't a good idea though. Those stats aren't something for us to compete against. Now that beta sites can exist in perpetuity, there's no reason to fudge the stats to make us look ready for graduation. That really benefits no one.