The homepage slider in ShowTime is designed to be a full-width slider that takes up the full width of the user's screen. It will scale up your images to fit the full width of the screen regardless of what size image you upload.

If you'd rather make it so it only fills up the width of the content container, it's a relatively easy change. Go to Online Store > Themes > ShowTime > ... > Edit HTML/CSS. Open the templates folder and look for index.liquid. Grab this code from the top:

{% if settings.use_slider %}
{% include 'home-slider' %}
{% endif %}

Move it to right below this:

<div id="content"><br />  <div class="container">

This will make it so the slideshow only takes up the width of the content container.  You may wish to also make some styling updates to your style.css.liquid file in the assets folder once you move the slider.