The limitation of a FXT file size that can handle MT4 and the mql language was 2GB up to 2013.
It is now set to 4GB (since may 2013).
This limitation is still unclear. At the beginning it was due to the FAT32 format that was limited to a certain size for the reading of files.
If the FXT file is too big, MT4 will stop the backtesting at a certain point. It may even fail to launch the test.