To be clear, Harper meant that he plans to add muscle, not just eat a bunch and get chubby. Quoted in the Washington Post:
It was kinda crazy that everyone was going a little berserk about me putting on weight. I was 235 coming in last year and it’s not sloppy weight. I’m not fat. I mean, I look pretty good. I don’t wanna say that, but I’m not gonna lie. It’s not like I’m out eating McDonald’s and Del Taco every night. I eat good, my mom fixes dinner every single night – baked chicken, fish – she cooks a great meal every single night. I’m working out hard and I’m eating right and I’m gaining weight. I think it’s pretty good.”
This is only a story because of recent reactions to players who reportedly looked puffy upon arrival for a new season. In the NFL, Eddie Lacy was thought to be fat because of an unflattering photo. He went on to rush for 1,184 yards and 11 TDs as a rookie this season. Then there was superstar outfielder Mike Trout, who some thought was fat one year ago. In 2013 he stole 33 bases and had nine triples.
In both of those cases it was much ado about nothing, and that is likely the case with Harper as well.