He's always been a stockier rider and has always had a more muscular upper body than some riders I could mention (eg Froome). I can't say I've noticed anything particularly different about him this season.
Even if he is carrying extra weight, it's not really doing him any harm - we may joke about how many times he finishes second, but oh look, he's top of the UCI world ranking, over 300 points ahead of GVA. OK, so GVA may have more actual victories so far this season, but you can't fault Sagan for consistency. And as has been mentioned previously, he will learn to turn those nearlies into wins, in much the same way GVA has done this season.
I still think he could win a GT one day, as Sean Kelly did.