Elle est plus maigre qu'élancée.

English Translation

She is more thin than slender.

lanky = ungracefully tall and thin

élancée = qui est de grande taille tout en demeurant mince et gracieux.

English Translation

She is more thin than slender.

Does not make much sense in either language.

I think the sentence pair would make more sense using rather and plutôt, instead of more and plus, respectively.

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I do not know if the nuance translates well in english, but in my opinion, the sentence does make sense in french. The reason neing what you mentioned in May: the positive connotation. While both maigre and élancé, are used to describe a thin person, using “maigre” tells that, in the eyes of the speaker, this thinness is being pecieved rather negatively, while using “élancé” indicates that, yes, they are thing, but there is beauty, or grace, in their thinness.
It is not directly stated in the dictionnary, but you can see that for maigre, negative words are used:
“Qui a un manque de chair […]”
“Qui est faible dans son genre, […]”
"Cette ciselure est maigre il va falloir la recommencer. " (I mean, it’s not good when you need to start something over, right?)
Whereas, élance:
“C’était un mec mince, brun, beau, élancé.”

So yes, it is probably not something you are going to be using at every occasion in everyday language, but it’s good to keep in mind, to either enrich your vocab when writing, or understanding the subtilities of a description in a novel