Hi there,
I tried to access your website but the link you have provided throws an error:

The Portfolio section is using the iSotope plugin to have that grid like functionality. That plugin calculates the height of the grids to have them correctly on the page, for some reason it does not calculate the height correctly for your case and it adds that space. So it is not a problem of padding or margin, it is a problem of miscalculation.
That might be caused by different reasons. One important thing is the version of the iSotope library which maybe is served the old version because of the cache plugin.
The other reason can be the customization you may have in your child theme. So kindly enable the parent theme and see if it is working for you.
And finally, the way the page is set. I would do as follows:
- Add a new page.
- Set the Page Template to Portfolio:
- Check the front end for the result.
Also, I would check for the image sizes of the Portfolio Feature Images and would upload the ones with the same size.
These are the tests I wanted to do if I could access your website.
Hope that helps.