David Haye is actually not small - he's a medium-sized heavyweight, with similar proportions to Muhammed Ali.
David Haye
Compare his height and reach to heavyweights like Boytsov, Chambers, Ibragimov, Frazier, Marciano and Tua.
During the past few years, Haye kept struggling to get his weight down to 200, because he knew he couldn't take a punch from men his own size at heavyweight. By his own admission, he moved up to heavyweight because he couldn't make cruiserweight anymore.
Haye won't fight a heavyweight contender like Dimitrenko or Arreola because he knows he'd be knocked out. So he's going for the one big payday against a Klitschko.