Even after earth being so big, why is the thinking power of human so small to discriminate?

To rephrase your question, I believe you are asking: "Even though earth is so big, why do humans stoop to the level of discrimination?"

Unfortunately, discrimination has occurred throughout history.  Globally speaking, humans have tried conquering other humans since the beginning of time.  There is always a group somewhere that hates another group for any given reason at any given time.  Sometimes these are isolated, localized events such as those on a school playground.  Others are extremely historical, such as the Holocaust.  As Dr. Seuss so aptly illustrates in his book, The Sneetches, there are always people who will dislike other people for petty reasons.

Why does this happen?  Maybe it is because the world is so big and there are things so much larger than ourselves, we find that we are uncomfortable with the feelings of insignificance we experience.  Look up into the night sky and see the stars--one gets a sense of awe and of smallness.  Some people have come to terms with this insignificance and live comfortably with it.  Unfortunately, others do not enjoy feeling small in this world.  In order to make themselves feel more important, they begin judging other people who are different than themselves.  It may be a skin color, a language, an appearance, an IQ, or a star on the belly--but there will be something a person decides to judge in order to lift themselves up on a higher mental plane.

Sadly, there has yet to be a solution to discrimination.  Every society faces it, and yet no society has ever fully eliminated it.  The best way to deal with discrimination is to remember that even if a person is trying to feel big in their own mind, it hasn't changed anything on the outside.  Discrimination has never stopped a determined person from accomplishing success.  The biggest danger is that discrimination breeds further discrimination.  Often, as seen in Sneetches, the person or group being discriminated against will retaliate in kind.  Soon, nobody can tell who is the victim and who is the aggressor.  Do not stoop to this level of discrimination, even in revenge.  After all, the discriminator is actually the small one, and the one who rises above discrimination is the successful one.  

The world is indeed very big.  Embrace it.