It really has nothing to do with the length of a post. I actually find long posts annoying and prefer people to be able to get their point across concisely.
In principle, you are right that the length of the post in itself is not a measure of quality. One of the most influential insights can be expressed in a few letters : E = mc2 ... But it still stakes the pages over pages to explain the meaning and context, before it really start making sense.
And a point well made, in a concise manner is definitely better than rambling on about a minor issue.
So let me then ask you, do YOU think some the post that earned you 30 merits is, in comparison to other posts, really worth 30 merits? What's then the yardstick to determine what it a quality post and should get merits and what isn't?