This is likely due to a size difference between the documents/artboards. Since strokes are given a set value (5px for example) their size can vary greatly, depending on the overal size of the document. A 5 pixel stroke on an object that's 2000px wide will seem much smaller than on one that's 100px wide.
I suggest you expand the appearance of anything that has a stroke.
Select anything you've applied a stroke to and go to Object > Expand...
The object on the left has been expanded, you can see that the stroke is now a physical path, instead of just a style, as on the right.