On this table, the 2010 volcano in Iceland rates only a 4. Vesuvius and Mt. St. Helens were both 5's. There are 4 6's including Krakatoa in 1883 and Pinatubo in 1991. The only 7 on the list is Mt. Tambora in 1815. (The only one before 1600 on the list is Vesuvius so it may be incomplete for the period before 1600.)
Since they are just giving whole numbers, each one ten times that of one number lower, perhaps Tambora was not exactly 10 times greater, but just in that range.
Peruse the Wikipedia article on the largest volcanic eruptions and Mt. Tambora is not at the top. Then again, see another on Tambora and you get another perspective. Wikipedia should be taken with a grain of salt—perhaps a truck load.