So I did some size comparisons to them coming out of the hatchery. On the sides, there are these bars. Pteranodon at most almost reaches the top of the second bar, Dimorphodon at most reaches halfway up the first bar.
Ergo, by height alone, Dimorphodon is barely half the size of Pteranodon, which is correct.
Also in the video in question, Dimorphodon does NOT have a shadow whereas both Pteranodons do. If the Dimorphodon was where we THINK it was, we should have seen it's shadow SOMEWHERE, but we don't see it AT ALL. So the Dimorphodon has to be either closer and/or higher up than it seems. It also is moving AWAY from the camera at eye level and above while the Pteranodon is moving TOWARDS the camera below eye level and up. This sort of perspective makes it a bit confusing but until we get more confirmation or see it in game ourselves, there's no way to know for sure, but given the evidence, it's most likely just a perspective thing.

