The recommended image size is 1200px to 2000px width, and 400px to 600px height. When switching to a mobile/small screen view there is some resizing and cropping that is done, so the image won't be identical. I would recommend, when setting up the images for the slide, to make them middle aligned.
You can always set Mobile and Desktop section height by going to slideshow option as shown in the screenshot below:
Go to Customize > Sections > Welcome' to view all the options related to Homepage Slider.