Giving background-color to body applying whole page. Why?

Answer 1

height you see in inspect is min-height That is equal height's element div, background-color change max-height That is equal 100% his parents(html).

For Example:

change min-height and run inspect and see result :

body {
   background-color: red;
   min-height: 200px;
}

Hello World!

So, You see height in inspect Change 200px; But color red cover whole page.