A line drawn with the pen tool in a new A4 document has a line weight of 1pt, black line and a white fill. When the 'D' key is pressed there is no change to the line's appearance, which is what I've always expected. When opening a 'Print-Large' document and draw a line the appearance is again 1pt, black line and a white fill. However when I press the 'D' key the line weight changes to 10pt! This proves to me defaults can be changed. There is no preset or template visible in the Illustrator application folders for 'Print-Large' so is this something that is written in the code or am I missing a trick somewhere?
'Print-Large' is simply the 'Print' preset with the dimensions set to trigger the 'large canvas' mode.
Since Large Canvas mode scales everything 10x, it's doing what you'd probably expect -- the default 1pt graphic style becomes 10pt.
I don't know why it would draw at 1pt before hitting 'D' though. It should use the default graphic style from the outset. Maybe some oversight in large canvas documents.
If you save the file that results from selecting 'Print-Large' to New Document Profiles, you can use it as you would expect and it uses the 10pt default graphic style from the outset (and of course if this isn't what you want, you can edit the default graphic style and resave).
You can change it by creating an object with your desired attributes, then alt-dragging it to the default graphic style. This will overwrite it.