The content of the liquid phase in this magma turned out to be higher than previously thought, and amounted to 16%–20%.

One point that I was trying to understand was that this 16-20% compares to the previous estimate of 9%. That's where the "twice as much" in the headline comes from. I found this in the newscientist link.

They found the reservoir is composed of 16 to 20 per cent melted rock on average – compared with a previous estimate of about 9 per cent – depending on assumptions made about the shape of spaces between solid crystals. That suggests the reservoir contains about 1600 cubic kilometres of melted rock, or almost twice as much as the previous estimate of roughly 900 cubic kilometres.