A PLN contains all plans (each storey), sections, elevations, details, views, layouts, publisher sets, attributes, project preferences, etc., etc.
A DWG is just one view of one plan, elevation or layout and usually only has 2D detail in it.
So there is now way they could be alike in file size.
When saving (save as) a PLN file there is an option to compress the file - this should be on by default.
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