The raw materials may be the same weight but available space is also a consideration. Also remember it is possible to construct structures in space that would instantly fall apart here on Earth. The solar panels can be printed thinner than paper but still maintain shape due to zero-G. Sections damaged by space debris can be scrapped, salvaged and reprinted robotically so little new material is needed.
Another possibility is bringing an asteroid of the correct composition into orbit to be robotically mined. This would reduce or eliminate raw materials needed to be launched.
You are correct that 3D printing is a great solution for building repair parts. That’s the real power of 3d printing.
If you built the panels as flimsy as paper in space, you’d still need to assemble them and erect them with fat space-suited fingers. Keep the printer warmed up!