I had the same problem with my G4. It works great in the winter when I need my warmer quilt and better sleeping pad, but now during the summer, I can't even fill up half of it.
I haven't used the pattern you found, but it seems like it is fine. I can't speak to the size of the finished product, but it looks like it was a good find. I'll be saving it and may try it later.
For me, I just changed around the G4 pattern and made the sizes a bit smaller, as well as changed the way the bottom was formed to make shrinking the sizes easier.