I find that just because a post is long, doesn't necessarily mean it warrants a huge reply. If I write a 2k word inner monologue while my character more or less ignores everyone around them, I'm not giving anyone anything to work with. But just by having my character ask a simple question like "So how did you end up in this mess?" I'm inviting you to write paragraphs worth of dialog and thoughts. So it might not be the best idea to constantly try to match another writer's length. And it might not be your fault you have nothing to write about.

As long as someone engages me, it's not difficult to keep the creativity flowing. Typically when I get stuck in an RP, it's because either I or someone else wasn't engaging enough. but if you feel you have plenty to write about and nothing you write satisfies you, maybe you just have writer's block.