Why is your left lung smaller than your right?
Your two lungs are not twins - the left one is built smaller on purpose.
The heart sits just left of the center of your chest, so the left lung makes room for it. It carves out a dent called the cardiac notch and keeps only two lobes, while the right lung has three. The right side ends up bigger and holds a bit more air.
Press your palm flat on the left of your chest and feel your heartbeat - the lung underneath has stepped aside to give the heart that exact spot.
The left lung is smaller because it shares its space with the heart, not because anything is wrong.
Knowing the lungs are uneven helps you read a chest X-ray or a doctor's explanation without thinking something looks broken.
Heart leans left, so the left lung steps back: 2 lobes on the left, 3 on the right.
Learn the idea and practice English at the same time.