Willis is a good actor with a normal physique who does movies with action, not a musclebound action star.
On a related note, it's sometimes hard to remember that Stallone started out as something of a critics' darling. Rocky (which he wrote) was a huge critical hit and Oscar winner. He wasn't a steroid case in the original, just a big guy who could box.
He followed that up with more "Indy-style" stuff like Paradise Alley and F.I.S.T. Neither of which had him as an action hero. He also did "Oscar" bait movies like "Victory" (directed by John Huston). When those movies underperformed, he started cranking out the Rocky sequels, pumping up, and his credibility suffered.
In fact, the original Rambo, "First Blood," was considered almost more of a suspense film than an action one. Same with "Nighthawks"
