The quality of a post is often a subjective thing, because not all people have the same criteria, and I think that is completely normal. Some things can be explained very well in 2 or 3 sentences, some other things require a lot more effort and time and we cannot generalize that a good post is one that has 3 sentences or 30 sentences. A wall of text never means quality if it doesn't make sense, and most users won't even bother to read such posts - but if someone is really good at presenting quality information, I'll always be happy to read his post and reward him with merit.

The reason why some users reward posts that are not one or two sentences may be that in addition to quality, they want to reward someone's effort and time required to create such a post. Of course, this does not mean that there are no quality posts that are short and do not deserve merit - each forum user is an individual who should make decisions based on their own thinking, we are not a collective consciousness.