The following is my views and ideas on Role-Playing, and may not apply to all.
Consider the nature of the posts: What is an introductory post, and what is a response?
With an introductory post, the writer is setting the scene, introducing characters, and writing about what that character is currently doing. It directly sets up the current event, and has a large impact on many of the details that would naturally form further events and scenarios. Anyone that enjoys writing will use as many words as they see fit to achieve the above. Granted, some writers are less skilled than others, and my not have a long and/or interesting introductory post (it can be short and interesting). This is writing at it's purest (in regards to role-playing), and allows the role-player to freely exercise their creative ability.
Once the role-players have established their characters, the setting, and acknowledged the current event or scenario, the posts may get smaller.
Most posts after the first or second post are responses. In a response, the role-player is obviously responding to the post before it. They acknowledge everything that is possible for their character to acknowledge, and have the character take reasonable actions. However, we have already established beforehand all of the details that make the scene. It is these details that have made the introductory post longer, so to state them again would be entirely redundant and uninteresting to read. To avoid this, we cut a lot of those details out. The writer has no need to constantly remind the reader about the size of the room, the clothes a character is wearing, or how dirty the bar is. When you take these details out, the post is reasonably smaller. The role-player may then add a few newer details to showcase new information the character has noticed as time passed. These details are fewer than before (everything
important ought to be written already), and usually help to more strongly establish the scene, and/or bring the various characters into interaction with each other.
After that point, one may notice the posts get even smaller. This happens because the details are even more extraneous and have less reason to be mentioned. One switches focus from the scene around the characters, and instead focus on the interaction between them. One is left to have the character notice details about the other character(s), and to write about what they are saying or thinking. This ought to take fewer words to accomplish, otherwise the post may seem too cumbersome for an interaction. I've always found it unreasonable for a role-player to rattle off a dozen actions and dialogue tags in a single turn of a conversation. It ruins the pacing and flow of the conversation and creates a timeline issue.
- From Time 1 through Time 10, Role-player A has stated the order of events. If Role-Player B contradicts this, it disrupts that order of events. Consider if Role-Player B replied to every action and dialogue through Time 1 to 10. What you have is two people interacting, where Role-Player B is responding to everything, and Role-Player A is ignoring them. In the following response, Role-Player A is forced to backtrack their character, and have them doing multiple things at once. You get a timeline that makes no sense whatsoever. On the other hand, Role-Player A can just continue to ignore all of the responses that Role-Player B made from Time 1 to 10, and instead respond to the actions taken at Time 11 and later. However, does that feel like a natural interaction between people to you?
In other words, to make a more natural, flowing character interaction, fewer actions and dialogue should be written. It allows for the responding role-player to properly respond, or even interrupt, agree, or disagree with what the other character is doing. Consider how real life interactions work. If someone is talking a bunch and doing a bunch of stuff, you're stuck listening and watching, unless you interrupt.
Role-Player B can also just write his responses to the actions from Time 1 through Time 10 in a way that fits naturally, but that's an awful lot of work, and gives Role-Player A complete control over those events-- might as well just co-author a short story or novel.
As a conversation between characters continues, there are fewer and fewer details to write about. The focus is mostly on the words that are being spoken. To continue to mention the characters same emotional state, same gestures, or the same smile or frown, becomes silly. The reader already knows the character is smiling, and will continue to smile until the writer mentions otherwise. It is also silly to have a character swing from mood to mood. If the character goes from happy, to upset every single post (during the same conversation), it's ridiculous. Wouldn't that character stop talking to that person? Maybe attack them? Hug them? Flip flopping between emotions for the sake of dragging out a conversation is not good writing, and it is definitely not good role-playing.
What eventually happens is that the role-players are left with a never ending conversation, or they stir things up, and move the scene forward. They make a new introductory post to introduce a new scene. As darkangel76 mentioned above, the length of the post should start to vary here. As you introduce another character, event, or scene, the post should get longer to reflect this. There is something new to explore, something new to write about, and there should be adequate description.
Basically, if the role-players are good writers, and good role-players, they will have an inconsistent post length. The length will reflect just what is going on in the role-play, and not the word count a particular person wishes to achieve.