Answer:
The inner planets are small, so they orbit the sun in close proximity to it.
Explanation:
The Inner Planets are Mercury, Venus, Earth and Mars. Geographically speaking, this is because they're closer to the Sun than the Outer Planets. The Inner Planets will also be hotter than the Outer Planets because all of them have a single solid surface in direct contact with their star.
Confusion arises when people try to coordinate spectral type for evidence for geologic activity - since stratified atmospheres like Earth's protect us from about 97% of solar radiation it's hard to quantify thermal conditions really accurately - but if you measure planetary albedo (how much sunlight isn't reflected away) you can get an approximation.
