question about 177 barrel for condor

Hi guys

Got a new 22 condor a few weeks ago and was possibly looking to get a 177 barrel for it. I was looking at the barrels on pyramyd air site and it said the velocity would decrease with a 177 cal barrel. My question if someone can answer is why would the velocity decrease with a 177 barrel? Any info would be appreciated. I am newbie and was wondering why this is so. Thanks. Jim