Cell phones have gotten much smaller in the past 15-20 years. although the design of the phones themselves has changed a lot, the main reason engineers have been able to make them smaller is that
a) the emergence of touchscreens has made keypads unnecessary.
b) better reception and transmission has made antennas unnecessary.
c) people spend less time on the phone now, so the phones can be less powerful.
d) technological advances have made it possible to make smaller, more powerful batteries.