Using PDFBox 0.8.0 (also tried on 1.1.0 and 1.2.1) to split files result in bigger parts than the original.
Concerned files were made from openOffice .odt documents in version 3.0.0 using openOffice pdf Export and then merging several copies with pdfsam (http://www.pdfsam.org/)
In joined eclipse project the test file size is 10 712 749 bytes for 2812 pages and the result file sizes after splitting in two at page 2300 are : 8 812 515 bytes and 10 701 142 bytes.
Using pdfSplit in command line as result we have all single result files bigger than the original. An example is also attached. An error tells the original file is corrupted, but we tried it on a file (using pdfsam and without using it) with no error and with similar result, so I think it's not related.
This issue seems similar to