AI files not placing at exact size

Hi,

I've created Illustrator files at the exact size of the InDesign pages I want to place them on so they just pop right in. But they're not coming in at the same size. I'm doing command d for place and then dragging the cursor with shift held down from the top left of the page to the bottom right of the page but it never comes out evenly.

Any idea what's happening?

Thanks!