I took a quick look at your code. There are several issues.
The main one (causing the endless div resize) is this:
@import "@arcgis/core/assets/esri/themes/light/main.css";
Should be in your main styles.scss. Not in the scope of your app.component scss.
Secondly, your app layout is not good if you wish a fullscreen map application.
Your styles.scss file should contain something like:
html, body { height: 100%; }
Then your app.component.html file mention a html-head-body hierarchy that is already in index.html.
This is screwing your vertical positioning.
Keep it simple like this:
{{title}}
<div id="viewDiv"></div>
<router-outlet></router-outlet>