When you download and extract any large file, here's how space ends up being used (this is true of any large download-and-extract system, not unique to Impulse):
- Initial Download: 2GB
- Temp Folder While Extracting: 2.8GB
- Destination Folder to copy to after extracting: 2.8GB
2+2.8+2.8 = 7.6 GB, just rounded to 8 for even-ness. The assumption with that 8GB is also that you're downloading, extracting, and installing to the same drive (i.e. the most potentially limiting scenario). After you download, unless you choose to download and archive, you'll reclaim the 4.8 GB from download and extraction at the end of the install process.