..29.7 MB. That is smaller than 4.5 GB right?

Right, you just downsized to less than 1% smile GB (GigiByte) is 1000 MB (MegaByte) - roughly speaking (actually a bit more).

Just to make sure, the libraries in the software will not make a difference. The point - in your case - are the items in the plan itself (the furniture list bottom left pane).

.. "fooled around" with modifying floor plans that I'd seen online using the program and saved those to my computer. If I were to delete those, would that help me with avoiding this problem as well?

If I understand correctly, no, that will not make a difference - at least not if you do not open many files at the same time.

But you may be in the clear now. That said, always good to save changes in a new file. That way you can always go back to the last one if you bloat your file by accident.