In reality which will be more heavy, a bag of cotton or a iron bar both measuring 100kg of mass?

You said 100kg of mass.. *sighs* mass, while not the same as weight, is, in the context of the question, the same. You did not specify different gravitational conditions nor did you state differences in velocity or acceleration. 100kgs of feathers weighs the same as 100kgs of grass.. or any other two materials because they are both stated as being 100kgs.
You need to rephrase your question if you want an answer appropriate to your thinking because my answer is the correct one for your question as it stands.