There may be some threads of interest and help about this (and printing tips and assistance) in the Index thread, mate.

Usually, it’s because Adobe Reader has a default of ‘Fit to Printable Area’ - untick or deselect this option, and also check to see if there any margins being used that could result in your image being resized.

This guide below explains it far better than I can (I think you can ignore the paper size content)…

’Why Don’t PDFs Print at the Same Size as the Original?'

https://askleo.com/why_dont_pdfs_print_at_the_same_size_as_the_original/

Good luck with it mate - hope it works for you 😃