Gimp by default imports a PDF at 100 pixels-per-inch (ppi) Your image correctly exported @ 300 ppi for the printer. The printer is only interested at the page size as reported by the PDF.
To check the PDF in Gimp set the resolution to 300 ppi in the Import from PDF dialogue.
You should check with your printer/publisher for problems at their end
edit: Gimp PDF is essentially a bitmap image wrapped up as a PDF. A better tool for sending to a publisher is Scribus, Image from Gimp, add the text in Scribus if possible. Plenty of info on using Scribus around. (and versions for OSX)
edit again. A closer look at your PDF viewer. It is set to 'Preview' which possibly explains the small size. Encoding cairo 1.16 - my linux is 1.15, should not make any difference although who knows with OSX.
To check the PDF in Gimp set the resolution to 300 ppi in the Import from PDF dialogue.
You should check with your printer/publisher for problems at their end
edit: Gimp PDF is essentially a bitmap image wrapped up as a PDF. A better tool for sending to a publisher is Scribus, Image from Gimp, add the text in Scribus if possible. Plenty of info on using Scribus around. (and versions for OSX)
edit again. A closer look at your PDF viewer. It is set to 'Preview' which possibly explains the small size. Encoding cairo 1.16 - my linux is 1.15, should not make any difference although who knows with OSX.