Hi,
Thanks for posting in our community.
Generally, the placeholder should auto-fill the inserted images, but based on our tests just now by making a similar template with Slider Master, it seems like we did reproduce the same situation as your description and attached screenshots that the selected image will auto-fit to the width of the placeholder rather than filling the entire image placeholder.
However, if the placeholder is a Picture Placeholder instead of the Content Placeholder, it will work in the way of filling the entire image placeholder automatically as your demand. And this is the difference between using a CONTENT placeholder (and insert using the image icon) and a Picture placeholder. The content placeholder will automatically fit the entire image within the placeholder, whereas a picture placeholder will crop the image to whatever size and shape the placeholder has been created.

However, it seems like you have already mentioned it was image placeholders included in custom PowerPoint templates for your organization. But from the screenshots provided, the place you tested with images is a Content Placeholder and that can be the reason. And kindly decide to change the content placeholder to the image one in Slide Master or manually crop the pictures without editing the company template if anything inconvenient.
Do feel free to post back if there have anything above unclear and Hope you a nice day.
Best Regards,
Mia