Mysterious behavior when using CSS "object-fit:contain" property on images inside a flexbox container having a specific height

Question

I've got a flexbox container in which I'm trying to display two images with drop-shadows side-by-side. I want them to take equal amounts of horizontal space even though the images differ in size. I'm using a container with style "display: flex" and using "object-fit: contain" for the images to cause them to scale. My code works if I don't give the container a specific height. If I give the container a specific height, such as 300px, the images scale down, but the drop-shadow appears at a distance from the image edges as though there's a box wrapping them. This seems odd behavior. Can anyone explain this odd-seeming behavior, and is there a way in which I can give the container a height and still get it to work?

Fiddle to illustrate: https://jsfiddle.net/Lej1a6vp/

html:

 <div class="container">
    <img class="image" src="http://via.placeholder.com/300x400" />   
    <img class="image" src="http://via.placeholder.com/400x300" />
  </div>

css:

.container {
  display: flex;
  margin: 1em auto 3em auto;
  width: 500px;
  height: 200px;
  justify-content: center;
  align-items: center;
}
img {
  box-shadow: 8px -8px 10px #00000080;
  height: 100%;
  width: 40%;
  margin-left: 1em;
  object-fit: contain;
}

Answer 1

BDB88 provided me with the clue I needed, and thanks for that. My objective is to make a responsive layout that will show the drop shadow around the images and not some ghostly outline that's at a distance, and I want to keep the images' aspect ratios. I also want to mandate a particular height for the container, and not have the images overflow outside of it. My use of "height: 100%;" for the img tag was what was causing the problem. Combining that with the "width: 40%;" was causing conflict because both requirements can't always be satisfied simultaneously. By changing to "max-height: 100%;" and "max-width: 40%;" for the img tag, I'm getting the behavior that I was after. The CSS is now (I made some additional edits to make the behavior more apparent when viewing and scaling the window to simulate larger/smaller screen sizes):

.container {
  background: yellow;
  display: flex;
  margin: 1em auto 3em auto;
  width: auto;
  height: 200px;
  justify-content: center;
  align-items: center;
}
img {
  box-shadow: 8px -8px 10px #00000080;
  max-width: 40%;
  max-height: 100%;
  margin-left: 1em;
  object-fit: contain;
}

Answer 2

If you set the height to auto it will make them the same width but height will be different, if you want them to be the same height and width then you have to use an image with the same aspect-ratio for this to work since it is not possible to make it the same width and height without cropping if aspect ratios are different. There is a #FutureCSS property called aspect-ratio: 16 / 9; in which you can lock elements to a specific ratio but it's still not available in all browsers;

    .container {
  display: flex;
  margin: 1em auto 3em auto;
  width: 500px;
  height: auto;
  justify-content: center;
  align-items: center;
}


img {
  box-shadow: 8px -8px 10px #00000080;
  height: auto;
  width: 50%;
  margin-left: 1em;
  object-fit: contain;
}