File size unusually large when exporting from Illustrator

I recently had to create an invoice for a client in Illustrator with minimal copy applied. The illustrator file was 361K, yet the Acrobat PDF which I exported using standard PDF/X-4:2010 (Acrobat 7) came to 2.2MB. That seems disproportionately large for such a small, single page project. Any thoughts on why?