How do people get so wide and big and tall?

I've always been average size I'm 22 5'7 150lbs and I'm curious because I seen people older than me that are like twice my size, like around 300 lbs and around 6 feet tall and it seems like almost everyone I see now a days is so much bigger old guys I work with are about that size and teens, and even most women. Like do people just not stop growing?
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Should I double what I eat?
How do people get so wide and big and tall?
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