I believe it's actually that Orks grow the more they fight. Hence why Warbosses and Nobs are larger.
It really depends on how massive you want to go here.
You could just use Ghazzy as a standard Warboss in mega armour.
You could get a
WHF giant, paint him green, add some dakka and call it a day.
Or you could do what this ingenious bugger did and convert one out of a soulgrinder and A killa kan.