I didn't think GSP looked any bigger last night. At the pre-fight presser he said was ~ 184lbs which is typically where he has been prior to previous cuts.
When talking about "size" against Anderson I don't see weight being the key factor.
It's Silva's frame; long body, long arms and long legs. Very similar to Carlos.
GSP has trained with Rashad Evans and bigger Olympic wrestlers. If GSP can get the takedown I don't see him having a problem keeping Silva down regardless whether Anderson weighs 205 or 220.