PSD Export from Illustrator dimensions wrong...please help

Hello! So I do MOST of my designing in illustrator. I do UV printing so I have to create spot layers for some projects. Often I try to EXPORT from Illustrator as a PSD file, so I can simply open the file in photoshop and add my spot layers. However, I have noticed that when I export as a PSD file and open it in photoshop the deminsions are not the same at all and I need them to be exact due to me sending them to a printer that uses the jig from illustrator size. What am I doing wrong?