Depends on file & filetype. Sometimes some filetypes reserve the space on the hard disk drive not long after you start downloading the file. Also, keep in mind LW does not download files in sequential order. It will download part of the beginning, part of the middle, part of the end, so ... this start & end portion of the file information is sent to the system which reserves it knowing the size of the file. As the file is downloading, it simply fills in the gaps.

With a program like Phex, you can actually specify to Phex whether to download the easiest found parts or rarer parts first if there are multiple hosts.

I have several incomplete files less than 5% downloaded which take up over 700 MB of disk space.