That sounds like the GEDCOM file was segmented with a program like GEDSplit. (For back when slow dialup was a major consideration. Dropped connections on a big files meant you started again from the beginning… and prayed.)
Are all the files of a similar size?
Usually these files were packaged with a small .exe to re-assemble the big GEDCOM file.
Although segmentation was part of GEDCOM spec… but was rarely supported.