Reduce Size of Vertical Images on Gallery Page

Site URL: https://www.eugenetheron.com/mountain

Morning, The vertical (portrait images) in my gallery always display a lot bigger than the landscape oriented images. Is there a way to make them similar sizes?

I have posted a link to my website, so that you can see what the issue is and also a screenshot below. The current website is built on version 7 but am currently building a new site on 7.1 and have the same issue.

FWIW - I have tried exporting them at different sizes from LR/PS and the issue still persists

Any advice much appreciated

Cheers