^It's just the angle and pose. See Haruna, her legs look really long!
And watch the Music Dragon performance, they use the same shoes, and their legs look fine! (actually, they look great!)
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On a related note:
After watching the Pis0/Tokisora lives endlessly after these past weeks (
), I've noticed that since the Tokisora outfits show the girls' legs very well, you can tell how well toned they are now.
Riho's incredible thighs are well known, but some others have gotten stronger as well. For instance Maachan, she was very thin when she started, and now look at the difference. Haruka also seems stronger (and Sayu said that she did improve her stamina as well on a recent interview, translated by randompaserby).
And it goes on, just check the girls and you'll notice.
If you watch old performances (easy, thanks to the NHK recent all single live 4 hour special), you'll see the improvement in musculature. Formation dancing is having some other benefits as well!