i bet alicorn are a link between all species, not just pegasus-unicorn, it's just that so many would forget this idea because she have wings and horn (well earth pony's are supposedly stronger, so since celestia and luna are taller this suggest they are strong like earth pony to ... and cadence is still young and not fully grown, same as twilight ) those are just deep trough i came in less than a minute ...
Why do people keep saying earth ponies are stronger? Rainbow Dash is probably number 3 after Applejack and Big McIntosh (not counting snowflake, because many bodybuilders can't use their muscles, because they have relative weak backs).
it is mostly because they do more physical tings wit their legs, as they only have legs to do tings. Pegasus can use their wings AND their legs to do tings so they mostly share the same usage of tings into 2 different type of muscles And you know focusing in building one type of muscle can develop them more than the concurrent who focus on building TWO types of muscles but can only use one type at a time (legs or wings to travel ...one is use at a time, instead of only legs for earth ponies).
Last, unicorn use their mind for most levitating tings so they don't train their physical muscles much ...
In result, the logical HABITS of each species make them more common to be of a type of build and capacity. it do not mean that one earth pony who never train will be stronger than the Pegasus who train and do exercise all day long ... but if both train as much, Pegasus will train wings AND legs, dividing his workout into two for each (approximately).
Wouldn't there be a bit of a continuity error there though? Since we SEE her flying about at the end of the episode... IN A DRESS! Which CAN'T be very good for flight. I mean, in A Canterlot Wedding, we SEE RD complain about aerodynamics regarding her own dress. So, there might have been a right proper cock-up with the developers there if that happens. :S
Since my first comment, I'd given the issue a bit more thought, and I might have a solution. I may need to view the episode again to clarify, but I don't think Twilight actually flapped her wings once during the little scene at the end. If I am correct, than I will say this: Twilight wasn't flying, per se, but was instead gliding. And I assume gliding is much easier than flying, I mean, even humans can do it with the right kit.
Nope, I just watched it again. Twilight actually flaps her wings all the way. Maybe she started by jumping off a tower to get high initial speed? (she's standing on top of a tower before the camera zooms out). One'd need a pretty high airspeed to pull off a complete loop (which she did) and as we've seen in Equestria Girls, Twilight can't even lift up or hover for more than two seconds.