Interesting data ! I've found that posts that are too long attract votes from established friends but not new readers. This suggests that they are getting automatic votes or not being read fully. With really short posts (which I don't do many of) it's totally random, depending on whether curation trails pick them up.
An interesting statistic, although I've got no idea whether it's possible to get the data, would be to look at payouts compared to the number of images in a post (allowing for the caveat that some images are just section dividers, "fluff" banners etc).
Not sure, need to take a deeper look is it possible