Definitely does not sound normal at all. Also, the memory allocated for skinned mesh updates shouldn't depend on the number of verts, but on the number of bones (or, depending on what's really causing the allocations, on both. But the only data that should be transferred per mesh each frame from Unity to the PopcornFX plugin are the transforms of the bones).
I took a look at the code to try to see what was happening, apparently it could possibly free and reallocate each frame in some corner cases.
Apparently there's an option to force the update of the attributes each frame, and if that's activated, or if the FX was hot-reloaded, or if the mesh was changed, it will free the bones buffer that's supposed to be reused between frames, and will allocate another buffer to transfer the vertices, then it will free that and reallocate a buffer for the bones and retransfer the bones, and will keep doing that each frame.
I'll poke our Unity-plugin guys on that on monday, they should know better why it's done that way, and if it really was intentional, if it's a bug, if there's a workaround, etc.