The problem with Samsung is , that they do not release their source code and most of the hardware in the phone I.e. the camera , ir blaster and the sound chip is proprietary , as Samsung owns the exynos chipset , unlike the US i9505 which runs the snapdragon core and has a gpe edition and a stable cm 12 , where atlest the phone is stable on the tried and tested snapdragon chip , and there is not much developer support for the exynos variant . So all Samsung phones on exynos chips will have lesser developer support as compared to their American snapdragon counterparts .
Pit files changes the system partition space available for the phone for system , bootloader , data , cache etc .
Standard Samsung tw roms require a lot of space hence the availability of 8.9 gb on first boot in stock roms .
On Chinese stock roms the tw is slightly trimmed , hence more space for data , so 11gb free on first boot on tw .
A pit file is basically a file that tells odin how to partition the phone .
Hence even if i9500 has cm installed , the pit file is the 8gb one . So a rom does not effect the partition data hence even with a 250 mb cm12.1 rom the size of the data partition will not change .
Now flash a Chinese 11gb pit file and flash a cm12.1 rm you will have 11gb data instead of stock 8.9 .
But if you try to flash back the stock international tw rom in odin , it will give an error of insufficient space .
So you will have to flash 8gb pit and then flash back the international tw rom .
PS playing with the pit file can corrupt your imei and permanently brick your phone . So not advisable .
Search xda for Samsung i9500 11gb pit .
Hope this helps .