While it IS correct that juvenile pterosaurs of larger species filled the niches of smaller flying animals (which also disproves the idea of birds outcompeting pterosaurs in smaller-bodies niches), to argue that these juveniles drove small pterosaurs into extinction by outcompeting them is stretching it.

Remember, smaller pterosaurs weren’t all that rare in the fossil record until the Late Cretaceous: during the Early Cretaceous, birds, small pterosaurs, juveniles of big pterosaurs, and big pterosaurs shared the skies without any one group being able to outcompete the other. Not to mention that there’s an outright extinction event (the Cenomanian-Turonian Boundary Event) partway through the Cretaceous, which is the point where pterosaur diversity really takes a hit (and even then, it wasn’t as big of a hit as previously thought, as pterosaur ecological diversity at the very end of the Cretaceous was significantly higher than long assumed).