Yup, Li-ion batteries have about 1/20th the energy by weight compared to gasoline IIRC. Now the battery packs weigh a bunch more than a full tank of gas, but not 20x more.
However, the fumes from burning batteries are FAR worse than the (admittedly also bad) fumes from burning gasoline!
I think that the batteries on the Tesla weigh about 1600 pounds. The battery (packs?) extend from the front bumper to the rear and are almost as wide, about 1 foot less than the width.
22 gallons of gas @ 6.2 per gal, equals slightly over 136 lbs. So about 12 times more. That is significant.
You are right, the battery fire is severe.