Speaking from my experience before as a trained dancer.
Putting on weight does impact your dancing ability, just like IMO it did for Ruka as well. While she may still hit (or pop) just as hard as before when she was slimmer, her extensions are not as clean and her lines are not as sharp anymore. Plus the weight will affect her stamina, which means timing may be effected since you are moving around with more weight now.
Check out her dance solo during the 4th gen auditions - still the best of all of her dance performances to date, IMO.