Ian,
With the physiological thermoregulation carried out by the larger python species, the heat is produced endogenously by muscular contractions and is therefore able to be very highly controlled within extremely tight limits. Live-bearers like anacondas and smaller egg laying python species, are unable to produce significant heat endogenously, and therefore must rely on spatially orienting themselves to absorb and retain solar radiation. While they are pretty good at temperature control through basking behavior, they still do not possess the degree of control seen in certain large python species.

Kelly