Constructive does not equal a lot of text, a short sentence that hits the nail on the head is also more constructive.
Brevity is the soul of wit.
If you are completely incompetent, you might say nothing at all;
if you are bullshitting, you could continuously spew away.
A well-crafted post will shave away the bullshit, leaving a simple message,
and that is much more effective than a long post, even if it's high-quality.
Perhaps beyond post quality is quality density: if substantial knowledge can be condensed across a brief reply.