I guess it's fair to say that the decision to post longer or shorter is up to each individual, and which choice is better needs to be evaluated on a post by post basis. I've read some really long, fluffy posts where I had to fight the urge to skim to the real point of it. I've read some short posts that would have been a lot better with more build up, descriptions, inner monologue, etc. There are also just times when a longer post disrupts flow, and a shorter post doesn't offer anything beyond an inner monologue parroting what all the other characters were thinking. 

Also, a lot of roleplayers just don't know how to write interesting descriptions. Two people can write an "info dump" on the history of a kingdom. One could pull you into the story and the other would put you to sleep.