I encountered to these kinds of questions when I was a child. As you are from Iran (home country), I know what you mean with "heavier". Consequently, the "equality" is the best answer.

Is one kilo of cotton heavier than one kilo of iron? No, they are both 1 kilo so they weigh the same.

Weight is a product of mass and gravitational force. If the mass of two objects are same and both are the same place on the earth then they should have the same weight.

Is one kilo of cotton heavier or one kilo of iron?

When you say an item is "heavy" or "heavier", you are talking about its weight, which is measured in Newtons, or loosely as its corresponding mass, which is measured in kilo(gram)s. The mass and thus the weight in the same place of a kilogram of each of these two substances is the same. Perhaps you mean "denser"? That refers to the mass of a given volume of a substance, which would be much greater in the case of the iron. People also use the word "heavier" to mean "denser" when the quantity has not been specified, so they might say "iron is heavier than cotton".