Mobile Header Logo Stretches

I followed the video for using Menu+ to add a "mobile header" with its own logo. In the video it was a simple 275x275 square with GP in the middle.

So I made mine a 275x275 square with my logo in the middle.

But when I set the mobile header logo to the 275x275 image I uploaded, it stretches it out... not the action shown in the video.

Here's what it looks like in the customizer:

How can I get it to simply show the square logo correctly (just like in the video)?

Thanks!