Overall it can be seen that population increased sharply by 4.4 billion over the course of 100 years.
As seen, you have very good attempt. Yet, you failed to introduce what's in the overview. An overview describe the general trend. As such, you'd better shun using figures or numbers in this.
there was a decrease
It is always good to use more "adjectives" in this part. Remember that IELTS is vocabulary testing. Polish your report essay with accurately sufficient lexical resources. This may be " there was a marked decrease", for example.
Asia witnessed a slight decrease in the amount of population by 6%
Let me give a try for this sentence; "The vast majority of the world's population (6 percent) resided in Asia witnessed a slight decrease."
You do not need to put a comma after while. This simple mistake deducts your score.
I am sure it is officially incorrect, since I cannot even find this word in the dictionary. Make sure you recheck spelling and grammar in use prior to submitting your report here.
Hope this helps, eddy suaib.