Progate html＆CSS道場初級がわかりません

画像のように全体的にサイズ感が小さくなる原因が知りたいです。

以下HTML

<html>
  <head>
    <meta charset="utf-8">
    <title>Progate</title>
    <link rel="stylesheet" href="stylesheet.css">
  </head>
  <body>
    <!-- ここからHTMLを書き始めてください -->
    <header>
      <div class="header">
        <div class="header-logo">Progate</div>
          <div class="header-list">
            <ul>
              <li>プログラミングとは</li>
              <li>学べるレッスン</li>
              <li>お問い合わせ</li>
            </ul>
          </div>
      </div>
    </header>
    
    <div class="top-wrapper">
      <div class="container">
        <h1>HELLO WORLD<span>.</span></h1>
        <h2>プログラミングの世界へようこそ</h2>
      </div>
    </div>
    
    <div class="contents">
        <div class="section-title">
          <h3>学べるレッスン</h3>
        </div>
        <div class="contents-item">
            <img src="https://s3-ap-northeast-1.amazonaws.com/progate/shared/images/lesson/html/study/html.svg">
            <p>HTML & CSS</p>
          </div>
        <div class="contents-item">
            <img src="https://s3-ap-northeast-1.amazonaws.com/progate/shared/images/lesson/html/study/php.svg">
            <p>PHP</p>
          </div>
        <div class="contents-item">
            <img src="https://s3-ap-northeast-1.amazonaws.com/progate/shared/images/lesson/html/study/ruby.svg">
            <p>Ruby</p>
          </div>
        <div class="contents-item">
            <img src="https://s3-ap-northeast-1.amazonaws.com/progate/shared/images/lesson/html/study/ruby.svg">
            <p>Swift</p>
          </div>
    </div>
   <div class="contact-form">
     <div class="section-title">
       <h3>お問い合わせ</h3>
     </div>
     <p>メールアドレス（必須）</p>
     <input>
     <p>お問い合わせ内容（必須）</p>
     <textarea></textarea>
     <p>※必須項目は必ずご入力ください</p>
     <input class="contact-submit" value="送信">
     
   </div>
  </body>
</html>

以下CSS

html, body,
ul, ol, li,
h1, h2, h3, h4, h5, h6, p,
form, input, div {
  margin: 0;
  padding: 0;
}

body {
  font-family: "Avenir Next", "Hiragino Kaku Gothic ProN W3", sans-serif;
}

li {
  list-style: none;
}

/* ここからCSSを記述してください */
.header {
  height: 90px;
  background-color: #26d0c9;
  color: #fff;
}

.header-logo {
  font-size: 36px;
  padding: 20px 40px;
  float: left;
}

.header-list li {
  padding: 33px 20px;
  float: left;
  
}
.top-wrapper {
  padding: 100px 80px;
}
.container h1 {
  font-size: 140px;
}
.container h2 {
  font-size: 60px;
}
span {
  color: #ff4a4a;
}

.contents {
  padding: 0 80px 100px 80px;
  height: 500px;
}

.section-title h3 {
  font-size: 28px;
  padding-bottom: 15px;
  border-bottom: 2px solid #dee7ec;
}
.contents-item{
  padding: 50px 40px 30px 0;
  float: left;
}
.contents-item p {
  font-size: 24px;
  padding-top: 30px;
}
.contact-form {
  padding: 0 80px 100px 80px;
}
.section-title h3 {
  font-size: 28px;
  padding-bottom: 15px;
  border-bottom: 2px solid #dee7ec;
}
input, textarea {
  padding: 20px;
  border: 1px solid #dee7ec;
  width: 400px;
}


.contact-form p {
  padding-bottom: 10px;
  padding-top: 30px;
}
.contact-submit {
  font-size: 18px;
  padding: 20px;
  margin-bottom: 30px;
  text-align: center;
  width: 400px;
  background-color: #dee7ec;
  color: #889eab;
}