Imported images not coming in a the correct size

Hi All, I have taken over a document from an ex colleague. It is set-up to iPad Pro (12.9in).

He had massive images in small picture boxes that, when exported to an epub file, is approaching 1.4gb. I am in the process of streamlining and organising this document to see if I can drastically bring the exported file size right down. I have set-up a grid of picture boxes at 624px wide by 334px high. These will contain thumbnail images and, when converted to buttons, will take you to another page. To populate these picture boxes, I have set-up a Photoshop document at the same size as the picture boxes of 624px wide by 334px high and at 300ppi. The PSD has 32 layers, so I have used the Export Layers to Files option, but the images pulled-in to the picture boxes don’t not appear at the sizes the images have been exported to. They are all 624px wide by 334px high at 300 ppi but come in at 149.76px wide by 80.16px high but with the actual and effective ppi being 300. Am I missing something? Should they not come in at the correct size at 300ppi?

If I use the Save for Web option, then images come in at the correct pixel size of 624px wide by 334px high, but at 72ppi. I get this as it is a Save For Web option etc.

I have tried this with a normal ‘print’ document and not a Web or Mobile version and I get the same results. Very strange.

Any insights into what is going on would be fantastic.