Really good topic, so I'll bite...
I think the Big Muscle body type is still the preferable physical "look" for Vince McMahon and the WWE, however, it's not given nearly as much priority or importance as it was in the past.
Simply look at the WWE roster during the 80's compared to the roster of today. Current stars like CM Punk, Daniel Bryan, Dolph Ziggler, Seth Rollins, Dean Ambrose, Cody Rhodes and upcoming talents such as Sami Zayn, Corey Graves, and Tyler Breeze are all being given or being groomed for prominent main-event level roles, but are far from the muscle-bound body types of past stars.
The wrestling industry (aka sports entertainment) has changed a lot from decade to decade, for instance as work-rate became more important heading into the 90's, we saw stars like HBK, Bret Hart, Jericho, Benoit, Guerrero and others given chances to be main-event level guys. What's happening today is just an extension/evolution of that.
Still, if a big muscled guy with solid talent comes along (like Roman Reigns) is the WWE going to push them to the moon as the "face" of their company? You're damn right they will. And there's nothing wrong with that. However, the door has never been wider for those talents who don't have the bodybuilder physique to become major players on the roster and that's nice to see.
The WWE understands that times have changed since the 80's and the bodybuilder figure is simply not as popular in society anymore. Today, it's more trendy to be trim and muscular, perhaps a similar look to your average MMA fighter. So in some ways, they don't have a choice. Also, the WWE is going to ultimately do what makes them money in the long-term, so if they are hurting financially and not pulling great ratings, they are simply more likely to push a guy like Daniel Bryan if he's getting a big reaction from the fans.
Basically, if the WWE is desperate enough, they will push an anorexic midget if they think it'll help get them ratings. Or on the positive side, today the WWE needs wrestlers that can work a faster-paced, higher work rate type of a match (compared to the WWE of the past), and that simply translates to guys who are slightly more mobile and not as weighed down by excessive muscle. Another way to look at it is, wrestlers like Roman Reigns, Cesaro, and Wade Barrett are considered "bigger/larger" guys in the WWE today, but would have been smaller-to-average guys back in the 80s.
So....yes, big muscles and size still matters to the WWE, but not as much as it used to.