Back in the 1980s, ENO publicity used to describe Rossini's melodies as "light-fingered" - one knew what they meant but (except possibly in the case of La Gazza Ladra) the terminology was unfortunate ...
I think Rossini primarily stole material for re-use from himself, so I don't think his lightness of finger is especially reprehensible?
I'm rather with JW here... I don't find "long-limbed" to be a particularly useful epithet for Bellini's melodies either!
Dessay and Callas are two extraordinarily different voices, though... I wonder what Bellini would have expected from his soloist in this role, and what kind of voice he ideally wanted? I like Dessay in lighter and coloratura roles, but I wonder if she really has the "guns" for Sonnambula? Or am I coming to this with post-Callas expectations? What I really mean by this is that I can't see how Dessay's voice - although I'm sure she can sing the notes - would have the dynamic and timbral range to keep the interest and display the spectrum of emotions involved? Maybe that can be doctored in the studio? But on stage?