YouTube lifts its skirt a bit on how a video gets to be considered popular over on its Creator Blog:
The best thing creators can do to be successful on YouTube is make videos that people want to watch. Simple, isn’t it? Unfortunately, many of the tactics we’ve heard from creators to optimize for YouTube’s discovery features may in fact backfire.
For example, we’ve heard from some creators who intentionally made their videos shorter in an attempt to get a higher retention rate. Unfortunately, this won’t help. While high retention on your videos is a good indication of engagement, we are actually optimizing for how a video contributes to a longer viewing session on YouTube. So your video isn’t more likely to be seen just because it’s shorter.
Well duh, try the opposite.
Conversely, we’ve also heard from some creators who intentionally made their videos longer, assuming that longer videos lead to more watch time. This also isn’t necessarily true, because it can be more challenging to keep viewers engaged through a longer video.
Once upon a time, these kinds of numbers were the business of studio or network executives who would use them to breathe down the necks of creatives. Now they’re the bread and butter of “creators.”
I often wonder why Vimeo has better content than YouTube even though the latter has the vastly bigger audience. My guess is that YouTube creators want stats; Vimeo filmmakers want to share a story.
Make better content. Screw the numbers.