Hasn’t she lifted objects this heavy before and had no problems? I recall her doing so in the second season. She throws tom out her top building. She isn’t a weak unicorn, just the cast members and writers don’t seem to remember or care. Just seems like anyone other than Fluttershy could have thrown that boulder. Applejack and Rainbow Dash are strong enough to throw it and Pinkie is Pinkie. During the fight with Cozy Glow, Rarity only levitated Pinkie, something that she probably didn’t actually need to do. Throwing a Rock seems like it is so generic for her to do. Isn’t this what Killer Croc does in the Batman cartoons? “I threw a rock at them!…. It was a big rock…”

@Background Pony #BD59  
Basically that. It’s a trope that some people do with characters that have psychic abilities to sort of balance things out with them and not make them too OP. Their magical “strength” can act similar to physical strength. So if they lift something heavy using telekinesis, it’s going to take more of an effort than lifting a lighter object.

@DragonRanger  
Some magical feats are actually difficult for some unicorns and has negative side effects, especially if they’re not really as skilled in magic.
 
For example, in Season 1 Twilight would sometimes appear dazed and confused after using the teleport spell. Something similar happened to Sweetie Belle too.
 

@DragonRanger  
Levitating a huge object and throwing it at a high speed likely takes a lot of effort.

@culix  
This scene actually confused me. Why was Rarity having trouble with her magic? It’s not like her magic had been drained.

loved this moment. rarity’s family line is pretty weak when it comes to magic but they are fighters =D

So many faces in half a second