Use the flex-grow Property to Expand Items

Summary

The flex-grow property controls the size of items when the parent container expands.
- If one item has a flex-grow value of 1 and the other has a flex-grow value of 3, the one with the value of 3 will grow three times as much as the other.

Final Code

<style>
  #box-container {
    display: flex;
    height: 500px;
  }

  #box-1 {
    background-color: dodgerblue;
    height: 200px;
    flex-grow: 1;
  }

  #box-2 {
    background-color: orangered;
    height: 200px;
    flex-grow: 2;
  }
</style>

<div id="box-container">
  <div id="box-1"></div>
  <div id="box-2"></div>
</div>