Have you ever wondered  

Good qusetion... I never thought of it.... Well! yes... why does a tractor have different sized tyres??... ...
It does not have big tyres infront like the back ones.. because...if it were then the driver would not be able to see what is there infront... he he......
And the back tyres are big.. as it gives the driver some height to see infront....
I dont know actually... I just guessed...

I really do not know what the exact answer is and I do not want to beat around the bush. I am looking forward for a person who knows the answer. So awaiting time for this topic.

The small front tires are for maneuverability and vision. Think of a farmer cultivating rows of young corn. He needs to see where the rows are. Those large back tires are for power.

Well thats a good question.It is so because in order to get more power the tyres at the back are big .And the front one is not that big compared to the back one because thats not needed for power and its free.Even if it will be big there wont be any use instead it will be wastage and the inertia of the wheel increase which will produce more difficulty for stearing.And the back wheels has to generate more power to overcome the resistance produced by the plough.
hope your question is answered

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Happy mylotting.