I think you have the Preview Preferences set this way where it defines "Actual Size" as Print Size and not Photoshop's 1 image pixel = 1 screen pixel. I tried it and my image looks larger. Check that Preview setting and try Actual Size again. There is nothing wrong with your image, just the Preview viewing settings. An image that's exported to jpeg may have the metadata taken out and the ppi goes from 300 to 72, which I think you will see when you check the jpeg's image size in Photoshop.
72 ppi would make a 4x larger size print proofed on screen.
So I think you have two things going on: Preview's definition of Actual Size set to Print Size and your PSD losing the 300 ppi print metadata and having it set to 72 ppi (if for example you set metadata to "none" on export).
At least that's what it looks like from my checks on the Macbook Pro and OSX 10.12.5
Thanks for the reply Gene. I checked Preview and it's set to 1 image pixel equals 1 screen pixel, but you're right when I check the image size it's gone down to 72. I checked the export settings and it's got Metadata options "none" or "copyright and contact", both of which give me 72 when I export and save. I've just tried to Save As instead of Export and it's saving as 300 now but still shows larger in Preview. Is this maybe just an issue with Preview then?