Logo shrinks when resizing the browser

The logo of a website that I am working on is great until I start to decrease the height of the browser. It starts to shrink. 

Here is the HTML code: 

<header > <a href="#" ><img src="images/eyada logo.png" alt="" ></a> 

And here is the SASS code: 

.header { height: 15vh; width: 100%; background-color: transparent; color: $white; display: flex; flex-wrap: wrap; justify-content: space-around; align-items: center; position: absolute; top: 0; right: 0; &__logo { height: 80%; padding: 1rem 0; &-img { height: 100%; } }