Arbitrary because A) BMI (the measure that this article and all obesity crisis statistics use) is a nonsense number that for instance penalizes you for being tall (hey guess what happens when children grow up healthy? They grow up taller) B) the number at which you 'become' obese has no real medical backing (partly because it's a nonsense number.
The American obesity crisis for instance is largely because we dropped our arbitrary threshold down to the WHO's threshold and gained some tens of millions of obese citizens in the process.
Sure, it depends on the person and their tolerance negative effect, but there isn't any fumbling with BMI or flatly checking a person's weight.