I don't think most posts are huge. Some of them are long, some others are not at all. Average posts are not very long, anyway...
In some threads is good to have people replying with more than a short post. It means they actually took some time to think and write down their opinions. But lenght is not that important. Some short posts can be very interesting too. They express in few lines what others said with their huge posts. Anybody should be free to express themselves in a way they find proper for what they want to say. As Chara mentioned, if you have things to say, just say them, and take all the time and space you need for this purpose. I'm sure around here there are people who avoid to read long posts or who just read bits of longer replies, but this way these people could miss some very important and interesting points, especially regarding complex discussions. If you reply, you must read the whole thread, long posts included
In a forum you have to be interested in reading and posting, also trying to put some effort in the way you express your opinions, I suppose.