Why Has God Imposed More Duties On Man Compared To On Women?

I don't think that it's the way you think it is.  Men do the 'heavy' work load because they are physically stronger. Could you see a little woman out there trying to do that, it would never be done. Women tend to the children, which is a job in itself, and keep the house in order, as well as some of the work outside the house.
Have you thought about how many children might have been in the house in those early times? There was no birth control other than abstinence, there were huge families, and the mother had to see to them constantly, and clean, cook, feed the animals, prep the food for storage, make the food because there weren't a lot of stores to buy pre-made foods.
There's a lot that the man did, but I think you underestimate what role the woman played in their lives together.

When God decided to create human beings, first of all man was created and then came the number of woman. As God wanted to take more work from man than woman so He blessed man with more power, wisdom and courage. So far your question that why God has imposed more duties on man, following explanation can be given in this connection.        
When a man grows young and starts earning, first of all he has to go through the responsibilities of his parents as well as his younger brothers and sisters because at that time parents had gone old and brothers and sisters are too young to earn their living. Later when he gets married more responsibilities are imposed on him from his wife and later from his younger children. Though woman cooperates man in every walk of life yet as far as earning is concerned, only woman cannot cope up with the expenditures of a house. If the woman works, she will only look after her husband or children while a man has to look after his parents, wife, children as well as his relatives. In simple we can say more responsibilities and duties are related to man, that's why he has to work harder than woman.