Hello @HunterZ and welcom to the forum!
Block-level deduplication makes it sometimes impossible to say how much space a file adds to the size. Additional copies of the same file add roughly nothing to size of backup. Partially-identical files add some.
Compression is sometimes a factor, but for many people the largest files are audio or visual media, and Duplicati doesn’t try to compress those again when it builds the zip file if they’re in a compressed format.
Versioning can apply. 10 edits of a photo might take 10 times the space. Text appends might deduplicate.
Having said all that, there is a wide selection of tools for your computer that can guide you at least a little.
Here are some examples, and as local tools I suspect they can visualize better than Duplicati’s web can.