Css – Display flex seems to be limiting the width of a component

Im making a reactjs app that so far got a header and another component. Initially it seems ok, but when i add a display: flex to their div, it seems to shorten the width of the header, as shows the images:

How can i fix this?
This how the codes are looking like:

function App() {
  return (
    <div className="App">
        <div className="wrapper">
          <main>
            <Header />
          </main>
          <Player />
        </div>  
    </div>
  );
}

export default App;

And the css:

*{
  margin: 0;
  padding: 0;
  box-sizing: border-box;
}
:root{
  --red: #990606;
  --black: #222224;
  --white: #f0f0f0;
}

@media (max-width: 1080px){
  html{
    font-size: 93.75%;
  }
}
@media (max-width: 720px){
  html{
    font-size: 87.5%;
  }
}

body{
  background-color: var(--white);
}

body, input, textarea, button{
  font: 500 1rem sans-serif;
}
h1{
  font-size: 2rem;
  font-family: 'VT323', monospace;
  font-weight: 500;
}
h2{
  font-size: 1.5rem;
}
button{
  cursor: pointer;
}

.wrapper{
  display: flex;
}

.wrapper.main{
  flex: 1;
}

Header jsx:

export function Header() {
    return(
        <header className="headerContainer">
         
        </header>
    )
}

Header CSS:

.headerContainer{
    background: var(--black);
    height: 6.5rem;
    display: flex;
    flex-direction: row;
    align-items: center;
    padding: 2rem 4rem;
    border-bottom: 1px solid var(grey);
}

Then the player code:

export function Player() {
    return(
        <div className="playerContainer">

        </div>  
    )
}

And its CSS:

.playerContainer{
    padding: 3rem 4rem;
    width: 26.5rem;
    height: 100vh;
    background: var(--red);
    color: var(--white);
    display: flex;
    flex-direction: column;
    align-items: center;
    justify-content: space-between;
}