Warne and Prasanna are the two real fat spinners I recall from the greats I have seen but most others were trim enough in their playing days. Bedi, Venkit, Chandra, Durrani and Nadkarni from India, Underwood from England, Gibbs from Windies, Mallet from Australia, Naseem-ul-ghani from Pakistan come immediately to mind from the plder lot.
Kumble, Vettori, Saqlain, Harbhajan also cant be termed fat.
I suppose they are a bit laid back and unwilling to put in much physical effort to start with which makes them gravitate towards spin in the first place