The bag of iron is heavier than the bag of cotton though they both have a mass of 100 kg, because iron is more densely packed than cotton. That is, the atoms of iron are more densely packed than cotton, and therefore, the density of iron is more than that of cotton and is much heavier than cotton.
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An iron bar  weighs  100 kg. 
A bag of cotton weighs  100 kg.  on the same weighing machine.

There is atmospheric pressure due to air around us.  

Reading of the weighing machine
   = The force exerted on the platform of the weighing machine
   =  weight of the object + force due to atmospheric pressure on the surface of the object.

As the volume and surface area of the bag of cotton is more than that of iron bar,  it means that the atmospheric air exerts more force on the cotton bag.  When the final reading is same for the iron bar and the bag of cotton,  it means that actual weight of the cotton bag is less than the weight of iron bar.

So iron bar is heavier.

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