It's nice to see you here in the Community. It will be my pleasure to provide clarification about the difference between the Layout Designer view and Print Preview when editing your invoice template.
Rest assured, the difference that you're seeing is working as designed. The Print Preview's main purpose is to show you how your invoice look like when it's printed. On the other hand, the Layout Designer is only meant to help you adjust the dimensions of your invoice so you'll be able to achieve what you saw on the preview.
This is the reason why the print version appears lighter and with a larger font. For more information about this, I recommend following this article: Use and customize form templates
Please know you can always reach me here if you're referring to something else, I want to ensure you're taken care of properly. Thanks for coming to the Community and take care.