Your slide textual content is inconsistent between the slides - hence the issue. So you can force it with this CSS:
.g-owlcarousel-showcase .g-owlcarousel .g-owlcarousel-item-image {
min-height: 60vh;
}
Adjust the 60vh to suit your needs.
To create a custom CSS compatible with Gantry 5 please read this
http://docs.gantry.org/gantry5/tutorials/adding-a-custom-style-sheet
.
Remember to recompile CSS from base outline too.
Regards, Mark.
p.s. putting textual content in the slide image is a bad idea (as you will see if you look in different viewport sizes).