Wilder's never really been massive, he's just a really lanky dude with a huge reach. His camp probably kept him light because they want him focusing on his movement, Fury is an awkward customer for anyone.
Also if the winner of this fight doesn't face Anthony Joshua then the whole thing is a farce. They're trying to do the whole Mayweather/Pacquiao thing and trying to build up interest but there is already a huge demand for it. I mean after Whyte and Usyk, who does AJ face? The division is paper thin.