Note that the Intel® graphics controllers uses shared memory that is taken from the RAM memory installed on your computer.
The feature that sets the amount of memory that is used for video is called Intel® Dynamic Video Memory Technology and this will make the graphics controller only to use the amount of RAM memory that it requires to run the different applications or software’s.
I am afraid to say that there is no way to preset this to a specific value; however, you can limit the maximum memory that it can take only. You will see this option usually in BIOS and you will have 3 options, 128MB, 256MB and maximum DVMT. Other values can be found as 512MB, etc but this depends on the motherboard manufacturer and the amount of RAM memory installed on your computer.
For your Mac system, try the above but if is not possible you must contact Apple for them to help you changing this setting.