There are a couple of different ways to measure how “big” something is. The first is an object’s mass (how much matter it contains) and the second is its volume (how much space it takes up). The smallest and largest planets in our solar system are Mercury and Jupiter, respectively.
The smallest planet in regards to both mass and volume is Mercury—this tiny world is nearly 20 times less massive than Earth, and its diameter is about times 2 ½ smaller. In fact, Mercury is closer in size to our Moon than to Earth.
The largest planet in our solar system by far is Jupiter, which beats out all the other planets in both mass and volume. Jupiter’s mass is more than 300 times that of Earth, and its diameter is about 11 times larger than Earth’s diameter. (Jupiter's Great Red Spot, even at its current diminished size, spans 15,900, just over a full Earth diameter.) Jupiter is 2 ½ times more massive than the rest of the planets in the solar system combined.