Thanks for using Duplicati and posing this question, wgstarks!
Is it possible Duplicati is following symbolic links in a way that whatever tool you’re using to check source size doesn’t follow?
For an initial backup (so no version history yet) the only way for the destination to be larger than the source is there’s no compression and no deduplication (either due to file types or parameter settings).
If those two things happen then the natural overhead associated with archive files (even with zero compression) and Duplicati’s related index files would make the destination SLIGHTLY larger than the source - but this is likely a few megabytes here, not the gigabytes you’re seeing.
There have also been a few scenarios identified where the source sizes in the progress bar appear to be “off” - do either of these topics match up with your experience?
(I know this second one isn’t specifically about the progress bar, but I thought I’d include it just in case.)