Does body mass have a bearing on reflex speed?
I went into this with the same assumptions that jello did, but I found two studies that had some interesting results.

Isojärvi (2010) found that in obese and under-exercised adults, physical fitness level predicted, among other things, a significant proportion of the variance of nerve conduction velocity and F-wave latency.  So, these individuals may theoretically have a slower response to noxious stimuli.

While Type II diabetes is not always a result of obesity, there is often a high correlation between the two.  Oltman (2005) found that, in "Zucker diabetic fatty" rats (an animal model of Type II diabetes), motor nerve conduction velocity in the sciatic was significantly slowed.  This effect, too, would affect the rate with which an individual could withdraw a limb from a noxious stimuli.

So, I don't think that anatomical changes in the bodies of individuals who are obese would have any effect, but clearly there are physiological changes in the neurons of these individuals.


References:

Isojärvi H, Keinänen-Kiukaanniemi S, Kallio M, Kaikkonen K, Jämsä T, Korpelainen J, Korpelainen R. (2010). Exercise and fitness are related to peripheral nervous system function in overweight adults. Med Sci Sports Exerc. 42(7):1241-5.

Oltman CL, Coppey LJ, Gellett JS, Davidson EP, Lund DD, Yorek MA. (2005).  Progression of vascular and neural dysfunction in sciatic nerves of Zucker diabetic fatty and Zucker rats. Am J Physiol Endocrinol Metab. 289(1):E113-22.