Yeah I do agree with your first comments. Size (bodyweight in kg's) should be last on the list of selection criteria, and some selections (or selectors) do seem to be bewitched by centres or wingers (or props) who are massive. However size is only indirectly correlated with strength, having more muscle only gives one more potential to be strong (larger and sometimes more motor units to recruit per unit of time) and a little more padding to help deal with contact injuries. For example, Neither Franks brother is massive by the standard of test props, maybe 110-115kgs but having been trained appropriately and consistently for strength (with an olympic lifting coach) for a decade and a bit has resulted in the 5-10kg they lack against the Ma'afu's of the world being irrelevant, and in some cases an advantage when you factor in their technical expertise. This probably also applies to Dan Palmer and maybe Benn robinson. Further out, you'll note that given that technical skill and courage is present, strength can be the ingredient that allows one to fully maximise potential. For example Digby is only about 95kg, not really big by test standards these days (think North 110kg, Savea 107kg etc.) but is immensely strong and it allows him to display his skill set (footwork, aggression with the ball in hand and the ability to make effective carries even under fatigue) well. McCabe, Juan De Jongh, BO'D are other example of players whose skill is paramount, and there ability to show it is enhanced greatly by physical strength. I think the prevalence of massive backs is more a function of a) all things being equal, the bigger man (appropriately trained to maximise his strength) will have an advantage or more likely b) skillfull and tough players are spending long off-seasons building and learning to carry bodies which make them imposing.
My attitude is that size on its own is irrelevant as a selection criteria. Specific strength is absolutely valid (i.e performance in maximal squat, clean, Deadlift and bench pull for props and possibly maximal single leg squat, Deadlift, bench press and bench pull for centre/wingers). All things being equal the stronger (not necessarily bigger) player will be advantaged, and since strength is the most trainable physical quality in an athlete, deserves due consideration in terms of training time.