I wouldn't say that was a fair statement. Rats come in a variety of sizes just like people due to genetics first and foremost and then how well they were fed /cared for in the developmental months.
Its possible that blues used to be bigger since they were strictly breeder rats not that long ago, but then they got leaked to the petstores and people breeding them wantonly wanting to make a buck, which deteriorated the quality of the blue rat.
Oh. I ask Becuase I've had 3 blues and all were bigger than most of my other rats. 2 girls 1 boy. 1 girl I've got at present is really big. I'd show a pic but I've no camra.
I've only had 1 rat that was bigger a black hood named Zepher. (r.i.p) You know simalar to the other name for a blimp. He was huge! I mean you had to use both hands to hold him! He wasn't fat or anything just really,really big. Maybe just a little fat but I think It was just boy fat you know how boys have that extra fat. Most of the time....