When importing one project into another, Premiere is not creating any new media, just duplicating the icons in the Project window from the imported project…just icons.
I understand how having all these new icons can cause you problems if you are not well organized (using bins) in both projects, otherwise you end up with a ton of stuff in the root directory which scrolls for miles.
Premiere’s Project file’s file size can cause problems for some. Primarily memory problems. If reopening your latest project takes more than a minute or exceeds 30 MB is size…save often.