Size Discrepancy between Master and Backup

I created an archive iMovie library on an external hard drive (called V1) containing completed projects from prior years. I then copied (using Finder) this Archive Library to a second external hard drive (called V2) so I could store it off-site. I recently added 1 new project and 2 updated projects to my V1 Archive Library, and when I went to copy it to V2, I found a serious size discrepancy in Finder between the 2 libraries on V1 and V2. I was expecting V1 to have a slightly larger file size than V2, reflecting the 3 updates. However, V2 shows its library size as 232GB , while V1 is only 119GB. I thought V1 must be compromised but they both have the same projects, except for the 3 new ones (which are relatively small) in V1. Before I replace the V2 library with the updated content in V1, I throughly went through the package contents in each library. While date/time created/modified/last opened are identical between V1 and V2 on all projects (except the new 3), some of their sizes are vastly different on projects that have not been touched on V1. In particular, the Render Files folder on one project is 26GB on V2 and only 205MB on V1 -- but neither file has been touched since I archived it to V1 originally. This doesn't seem possible if the files haven't been updated or even accessed in iMovie. What do Render files do? Is it possible that these files can become compressed? Any insight would be appreciated.