Why is the ruthless totalitarian regime, described in Orwell’s novel purging Oldspeak and imposing Newspeak? What assumptions are being made about the relationship between language and thought?

The aim of the Newspeak is to decrease the words and language being used. They would like people's thoughts and mind to be in control such that it is made easier for it to be understood. The conclusion derived from relationship with thought and language is made smaller from the diction used, the less the thoughts of people will become.