I've saved a two-colour Illustrator graphic as a jpeg, 600px width. Then opened it in Photoshop, where it opened at a width of 1270px. Easily rectified, I know, but why would Photoshop open a file from sister software inaccurately? Isn't compatibility supposed to be the whole point of the Creative Suite?
When you save a JPG in Illustrator, you can choose the DPI (dots per inch) that you export it as. Illustrator defaults to 150 dpi for its exports. When you export at 72 dpi (screen dpi standard), you will be able to import it at the same size.
Try to export the graphic from Illustrator but before exporting, choose 72dpi and then open it in Photoshop and see how it looks.
Maybe you exported with a higher dpi, and photoshop resampled the image with the dpi that you export from illustrator.