edit: After posting the examples below I found the answer posted here:
Gerald the answer seems that zenfolio does not dynamically resize the pictures. It creates several fixed sizes then picks the size that best fills the space. It's been that way since at least 2009 and it does not look like zen is changing any time soon. You can live with it, work with it (as zenbrian does in his landscape gallery but making every picture the same size with black bars on the size for the portrait orientations) or find another solution that resizes better.
Here's what I originally posted as samples:
I noticed this on the first gallery I uploaded to zenfolio.
You can see it in your own gallery:
Viewed in chrome, with the browser window expanded to the full screen and the resolution of the screen set to 720p one of the above pictures fills the available vertical space the other doesn't.
At 1080p on chrome there is so much wasted space that it's hard to see the difference but there it's flipped and the landscape fills less of the screen.
(the extra wasted space in 1080p may have resulted from changing resolution while the browser was open, perhaps it did not dynamically resize).
Another example is here (viewed browser at full screen res at 1080p)
Look how well this pictures fills the vertical space:
Beatiful. But then look at this the picture gets lost because it is resized differently on the vertical axis: