whe it happens just stop the download and reboot your pc...the download and install process is never faultless, so the pc usually needs a bit more free space to move everything eround...if you turn off your pc though the temp files he was storing annd are now useless are gonna be canceled, and so the space requirement will be as advertised..
notice that you have to repeat this process more than once depending on howlittle space you have in advance, but it should work..it does on uplay when downloading those big ubi games and i suppose on steam too.
Ah ok. Thanks.