A head cannon is that they have different disabilities. Bulk bicep is really just smaller wings, but scootaloo is actually magic deficiency. Scootaloo probably just don’t have the magic power to do it. I saw a head cannon that Pegasus, earth or unicorn have different magic core, and scootaloo got none Pegasus one. Whether or not that’s plausible, we can see some unicorns have weaker power than other, applejack are better at farming than pinkie pie with trees. Also that Pegasus have denser bones than bird but smaller wing span comparing to the body, to lift a horse you need so much more wingspan that their tiny wings.