My recollection of theory is many mammals (that we know survived to evolve) at the time of the KT crash were tiny, and burrowers and able to use both of these (

My recollection of theory is many mammals (that we know survived to evolve) at the time of the KT crash were tiny, and burrowers and able to use both of these (less exposure to damage from initial strike and need less resources to survive/procreate) to out survive the dinosaurs.