Why new jet aircraft have wider, larger cockpit windshield/windscreen?
The area of the windscreen is limited by the structural strength of the materials. The windscreen must withstand the difference between cabin pressure and outside air pressure during cruise. In addition, it must also survive impacts from birds and hail to a certain degree.

As material engineering advanced, we can produce larger windows and windscreen. For example, even though the Boeing 787 has a stronger pressure differential (6,000 feet cabin altitude vs 8,000 feet in traditional airliners), the passenger windows on the 787 are larger than previous productions.