It's not an issue with Sphax. It's an issue with what's he's doing.
Download the Vanilla pack from here:
http://bdcraft.net/download-bdcraft-texturepacks
Then download the patch and extract it to your original Vanilla pack
My server has around 10 at a time and it used about 3 GB with about half a gig to spare.
This is with a lag plugin unloading unused chunks all the time.
Follow the steps listed here
https://help.ea.com/article/how-to-gather-dxdiag-information
Instead if blabbering to EA post that information to Pastebin and leave a link here.
Your modpack.jar isn't properly configured. Download forge for whatever version of minecraft your pack uses and rename it to modpack.jar
Make sure the file extension changes