Steps to reproduce the issue

Make sure Protostar is your default template, and loads modal library.

You can do this by adding a line of code to templates/protostar/index.php (or to a duplicated template based on protostar):
JHtml::_('behavior.modal');

In a module or an article insert a link pointing to a big image (ideally wider than high) and with modal class:
<a class="modal" href="path/to/image.jpg">Modal link</a>

Expected result

In mobile devices, when you click the link to open the image, it should load fine keeping the aspect ratio.

Actual result

The image loads with 100% height and width, losing the proportions (the image is stretched):

System information (as much as possible)

Firefox 50.0.2
MacOS Sierra

Additional comments