Not necessarily. A larger car can have bigger crumple zones. If its crumple zones are twice the size of the small car, then the acceleration that you'll experience in the collision is a lot less and so there's a greater chance everyone will survive (assuming that the relative impact speeds will be the same).
Don't let facts get in the way of a good story. :) While survivability is about equal for SUV vs SUV and car vs car impacts, studies have shown that in SUV vs car impacts the passengers of the car are 7.6 times more likely to die.
Armed with this information an autonomous vehicle trying to protect everybody should: (a) choose the impact with the least inertia for all concerned (i.e.: go for the car travelling in almost the same direction as the autonomous vehicle as opposed a car travelling in an opposite direction) and (b) for a choice of head-on impacts, prioritize impacting the car with a mass closest to its own. An autonomous vehicle biased towards protecting its own driver should target the smaller vehicle... but this will inevitably lead to "I've got the biggest autonomous vehicle" wars with people trying to protect themselves from other vehicles as we've seen happen with SUVs.