I've noticed the same thing on these forums; longer intro posts (and sometimes even activity checks or 1x1 advertisement posts) tend to get less replies than shorter ones. Dunno why, but I guess it's a combination of people being lazy readers to feeling pressured, like you mentioned? I've no idea, because short or long posts don't really bother me and I haven't seen that kind of thing happen elsewhere. I mean, I originally come from forums where what's considered "long" here is average over there (I'm talking 1500+ word posts, which sometimes might go up all the way to 4000. Yeah.) and I've never felt pressured to match other posters' length or whatever. 

I've also done shorter roleplays with < 500 words, sometimes even < 200 words on messengers and stuff though  and, eh, that's never been a problem either. Although I _have _sometimes purposefully cut posts shorter if they go on for too long and others post shorter ones - precisely to avoid people crying over "not being able to match". Which is a stupid concept anyway; you write for as long as you feel like you have something to say and then stop. Done. Easy.

tl;dr: I've noticed the same and don't really get it, because length really doesn't matter to me. Long as there's something of substance to reply to - preferably with at least some character thoughts mixed in, because those are always fun to read - I'm golden.

... Also, yeah, I usually ramble like this, anyway.