Not sure if I would describe Boone as "noodly"
She's not slender, she's got a full figure. She has what some would describe as a "soft" bodytype. She's not fat, she just doesn't have an athletic build or a lot of muscle mass due to an early pampered lifestyle and little physical conditioning. (I'm not sure if psykers need to undergoe physical conditioning like regular soldiers, I would imagine no)
That and I don't think she had the option or motivation to exercise when she was imprisoned on a black ship and now she works as more or less a secretary if I recall correctly.