Inner plants are smaller in size and the rest of the outer planets are bigger, termed as Gas giants.
Step-by-step explanation:
It is a theoretical belief of all wise scientists that most of the universe is made up of Hydrogen and helium gases. These gases are the fundamental ones in creating the father of the Solar System i.e. the Sun and the other Giant planets such as Jupiter, Saturn, Uranus and Neptune.
The main reason why inner planets are smaller in size as compared to the outer ones is because of the substances of creation and their states. Most of the inner planets are composed of solid rocks which means, they are firmly bounded and have closely packed atomic structure.
On the other side, giant planets are made of gases which means their atomic configuration is much more spacious and hence they seemed to be greater in size although they significantly less mass.