What don't you ask the designer? We'd just be guessing.
The size of the ID file has very little relationship to the size of a PDF exported from it. For one thing, the PDF contains image and vector data that are merely referenced by links inthe .indd file. If the 24 PDFs are primarily vector data, and I'd presume the .ai files are as well, they won't compress (and the PDFs may not compress much even if they are raster).