As few thoughts come to mind which may or may not help you.
As mentioned by Old Toad, there is a potential for issues with Photo. Your post indicates that you have OS X 10.10.4 and I think that you would have acquired Photo with that version. If you ever opened it and "imported" your photos, then the links provided by Old Toad are essential reading.
Now, that said, I must also say that while I am expert in many things, not iPhoto. I tried both Photo and iPhoto and don't like them so take my comments as constructive efforts and not expert advice. Hopefully checking out a few of my ideas won't waste your time.
1. When you said you deleted the photos from the trash, I am wondering if the photos were actually in the trash. It appears they were not since the space changed by such a small amount.
2. As mentioned above, there is the issue of having Photo and iPhoto on the same machine sharing the same library. Since the links are hard links, deleting one does not delete the actual file. (See the article referenced by Old Toad.)
3. I have not experienced your situation, but what I would do is see if the deleted photos are in Photo even after being deleted from iPhoto. (The reverse is possible too, if deleted from Photo, it may be that you still see them in iPhoto.)
4. I personally tried to use Photo once and was confused when I "deleted" a large number of pictures and no disk space was freed. I then realized that "delete" just moves the pictures to a staging area and you must find the appropriate buttons to really delete them.
5. Having said all that, I wonder if all the thumbnails and edited versions are deleted along with the originals when using two different programs to do two different sets of deletions in a shared library. Someone more expert may know that and it may not matter much for your needs.
As you may already know, the originals for the images are stored in the library (with a name like Originals or Masters) of Photo or iPhoto in the Pictures directory. You need to "view package contents" to get finder to look into the library or use the Terminal and good old fashion Unix commands. If you decide to delete the pictures yourself, be very very careful and back up these libraries somewhere until you are absolutely certain that all is working to your satisfaction. (Of course, messing with cleaning out any files should be preceded by making sure you have an up-to-date back up of everything. For me, that is more than just Time Machine. It is a full back up of everything to an external drive.)
Good luck and let us know how it works out.