For the record I believe Silva's 215-220 lb walk around weight is when he is not in training mode and definitely not where he cuts weight from. There are many pictures of him when he is not fighting and he has noticeably more fat on him which he burns away in training camp. When he is training I believe he is naturally around the 200 lb mark just like we saw in his last fight.
GSP's "walk around" weight of 190-195 has also been stated as the weight that he cuts from. Even when he is not coming up to a fight GSP always seems to be in amazing shape. For example you can find a video of him having his body fat tested in his off season and still being 7.7%...
I think that is one reason you can't compare these 2 weights. If these guys were to fight at a catch weight of 178 lbs as previously discussed I bet Silva would be the same weight if not less then GSP before there weight cuts.