Barcode graphics are emailed by GS1 as .eps files.
This is a vector-based format to ensure any resizing of the graphic stays proportional.
How do I import an EPS file into Adobe Illustrator?
If you select "Link", Illustrator will create a link to the EPS file, and if you deselect it, the file will be embedded in the project.
When you're happy with the settings, click "Place".
How do I resize the graphic in Adobe Illustrator?
Select the whole barcode.
Right Click > Transform > Scale Options >
Scale Stroke & Effects [tick the box].