In Equestria it seems that a Unicorn's magic is limited by two factors: Their cutie mark, and their raw potential.
Twilight is
limitless in both of these categories. She was born better than everypony else (which is fair enough in the sense that some people really ARE just better than others)
Telekinetic proficiency doesn't seem to be affected by your cutie mark, but I assume your raw potential controls how much you can ultimately lift. Personal training is probably what determines how precise you are. Have we seen Rarity lift anything particularly heavy? I don't think so (The couch I suppose), she seems to mostly manipulate smaller things in much finer ways. She's VERY good at multitasking (she can man an entire STORE by herself using telekinesis), which makes sense given her profession.
Now, It's been argued (by me too) that Trixie's special talent is stage magic, but even for that, her arsenel of spells seems impressively varied. I actually think HER talent is the same as Twilight's, Magic itself (I subscribe to the theory that the same talent can be represented by lots of different Cutie Marks). However, Trixie appears to have below-average to average raw potential, explaining why she simply isn't as strong as Twilight. To compensate she utilizes smoke bombs and other trickery
Imagine what it must feel like to know that you SHOULD be one of the greatest equines who's ever lived (Magic has gotta be a pretty rare talent) but being stuck with a sub par body.
... Or I could just be talking out my ass!
![[lol] [lol]](https://static.tvtropes.org/pmwiki/pub/images/lol2v12_4863.png)
I just got onto a weird train of thought.
"Schneizel! So you ARE the
puppet master behind all this!" - Lelouch Lamperouge
Tealove is best pony