It sure seems like it to me. The power hitters don’t look strong anymore, just fat. Ryan Howard. Manny Shitmirez etc…
Am I imagining this?
Yes.
First off, Manny’s old. He’s gonna get fat. Second, Babe Ruth was a friggin’ whale. Third, you’re working on selective memory. For every Prince Fielder you mention, I can bring up a Cecil Fielder (okay, bad example). For every Ryan Howard, I can bring up a Steve Balboni. Or Carmelo Martinez. Or Dee Brown. (Okay - I bring up a lot of ex-Royals…)
Babe Ruth.
Willie Stargell.
There were actually players called Jumbo Brown and Fat Freddie Fitzsimmons.
A lot of power hitters look like softball players, big overly muscled goons.
There were a lot more fat players when I was a kid. Back in the Seventies, you saw all kinds of body types on an MLB team, from muscular to scrawny to obese.
The players are, on the whole, a LOT more fit, muscular and athletic-looking than they were a few decades ago… though baseball is STILL a sport in which a guy with a pot belly can succeed if he can swing the bat or get batters out.
A characteristic it shares with curling. 
Maybe it’s just the baggy sloppy way these guys wear their uniform nowadays. It’s on optical illusion.
“I’m not an athlete. I’m a professional baseball player.” - John Kruk
How old are you? If you grew up in the steroid era, you may not be imagining anything. The players who took steroids were much more buff. If you discount the steroid era, players nowadays are not any more or less fat than they always were.
Plus, Ryan Howard is not fat anymore.