Habit nowadays.
I used to post like, one paragraph of the character's thoughts, movement, action, and the like, followed by a line of dialogue spoken to the other character. Back then it was quite the weird thing, because while I do write on that first paragraph, my partner can pretty much just read the spoken line, and just write from that, and it'll be fine.
It wasn't until I do meet someone who loves write a long story, 2000 word minimum they said, and I felt that I need to up my writing's quality as well, because it felt like they were trying THIS hard, so I at least need to try a bit harder. It just grew from there I guess, I now average at around 600 words, two paragraph and one short paragraph, but could reach 5000 if needed, especially if I'm controlling two character that is interacting with each other (so basically, RPing with myself), and once I reached that threshold, writing anything less than three paragraphs felt lacking.
By no means that this is a requirement for my partner either, but I do prefer longer post just because I enjoy reading, and also typing I guess. I just feel like longer paragraphs can convey a lot more details (duh). I also like peppering the long paragraph with subtle hints as well, nudging the character's development, or the relationship development, forward. This is more of a treat for the writer, than something that the character could've felt or seen though, because me personally, I love finding those little nudges in the post, making me squee and happy inside, so I try to give that feeling to others as well.
Personally, I write in three paragraphs as more of a representation of the human mind. First paragraph would usually contains stuff that the character felt or thought regarding the response the partner's give, second paragraph would contain the processing part of that feeling and thoughts, and then the third part is the action and dialogue that my character give. This way, I could develop my character, the world around the characters, and give something to respond as well, and to look into if perhaps my partner would like to employ 'face reading' or 'gesture analyzing' part of their character.
By the end of the day though, I usually just let my hand and mind do most of the work. The post ain't done when the imagination ain't finished flowing.