Why you would compare a minivan to a compact SUV in terms of mass? The Sienna and Odyssey both weight ~4500 lbs. That's more or the same as the Grand Highlander or Pilot, both mid-size SUVs. Compact SUVs typically weight in the 3500-4000 lb range.

It's true that full size SUVs and trucks weigh even more though.

What is the physical mechanism that would cause a vehicle hitting a more massive vehicle to be worse than the same vehicle hitting a wall at the same speed? My understanding is the impulse is greater in the wall collision. It feels like a wall is the ideal test. I know there has been talk of getting the NHTSA to test offset frontal collisions, but it would still be stationary.

Regardless, I'm sorry that happened to your family. I would fear for mine if a similar thing happened. The arms race of ever more massive vehicles is an unfortunate social dilemma problem.