The Jewish population increased much faster than the Muslim (or Christian) population. Most of the growth in the Arab population can be accounted for by natural population growth, rather than by immigration.
Jewish immigration was encouraged by European leaders and European Jewish groups between 1900-1940. As Jews came in they developed irrigation systems, electrical grid, created farms and industry, and Arabs came to work the land and factories and build their own too. My main point, however, is that it is not "stolen" land, the Palestinians are not 2000+ year old people. They came to the land from different places and from different tribes in the Arab world just like the Jews came from the UK and Poland and Germany etc - and most of them roughly from the same period of time.