I don't think length intrinsically denotes quality. One could say that longer sentences show more quality, but I would not even be certain of that either.

The biggest difference I noticed has been between smartphone/tablet users on the one hand, and oldschool desktop computer system users. I belong to the latter group and I think we, as a group, write more, and faster. I'd also like to assume it has a higher quality, but I am not automatically convinced of that either.