German bombers so much tougher than RAF bombers?

[trademe900]

Even with the same plane versus RAF or German bombers, why is it that the Wellington or Blenheim will go down so easily but the He111 or Ju88 needs an absolute slaughtering from multiple fighters to go down?

With 5 fighters versus 20 blenheims at least half of the blenheims will be lost. The result is even worse for the wellingtons. For the same scenario hardly any german bombers are lost though.