20 80 width

two inline blocks aren't inline when their width calculations appear to be sound

HTML

<div id="main-container">
  <header>

    <div id="title">
      <h1>Welcome!</h1>
      <h2>Hello World!</h2>
    </div>

  </header>

  <section>

    <div class="col-small">
      <img src="https://source.unsplash.com/random">
    </div>

    <div class="col-wide">
      <img src="https://source.unsplash.com/random">
    </div>

  </section>
</div>

CSS

html {
    box-sizing: border-box;
  }

  *,
  *::after,
  *::before {
    box-sizing: border-box;
    /* The Box Model Fix */
  }

  #main-container {
    width: 100%;
    margin: 0;
    padding: 20px;
  }

  #title {
    width: 100%;
    height: 400px;
    border-radius: 5px;
    background-color: #193e7a;
    overflow: hidden;
    min-width: 900px;
  }

  #title h1 {
    /* font-family: 'Indie Flower', cursive; */
    font-family: 'Dancing Script', cursive;
    margin: 180px 0 0 20%;
    font-size: 100px;
    display: inline-block;
  }

  #title h2 {
    font-family: 'Indie Flower', cursive;
    /* font-family: 'Dancing Script', cursive; */
    margin: 0 0 0 20%;
    font-size: 30px;
    white-space: nowrap;
  }

  .col-small {
    border-radius: 5px;
    width: 25%;
    height: 300px;
    background-color: #a3b4d1;
    border-top: 5px solid red;
    overflow-y: hidden;
    overflow-x: hidden;
    min-width: 280px;
    display: inline-block;
    margin-top: 20px
  }

  .col-small p {
    padding: 10px 15px 15px 15px;
    margin: 0;
    display: inline-block;
  }

  .col-wide {
    border-radius: 5px;
    width: 80%;
    height: 300px;
    background-color: #a3b4d1;
    border-top: 5px solid green;
    overflow: hidden;
    min-width: 480px;
    display: inline-block;
    margin-top: 20px;
  }