The KT mass excinction killed basically everything over five kilograms. Things that should be noted are the fact that there was already an extinction event happening when the asteroid hit, due to a greenhouse effect caused by an excess in co2 in the atmosphere (sort of like what’s happening now) so everything was already dying. Also not everything died during the impact, it’s more like a slow devastating ecological collapse caused by the food chain collapsing. Smaller animals tend to have a better chance of surviving as a result of the food chain collapsing because in general they have lower calorie needs, so are just less affected.
That’s why all the non-avian dinosaurs died, not just the American ones near the impact zone. Because it causes waves of food and resource shortages across the entire planet.
Try to take a very long view of things, because an “extinction level event” is rarely an “event,” as you think of that word. There wasn’t one day where dinosaurs existed in the morning and were all dead by noon. Evolution is a process that takes place over l o n g periods of time.
So, how did, say, small mammals survive a meteor strike? Lots of possible reasons:
* They were on the other side of the planet.
* Their predators stopped reproducing at their previous levels.
* The food they relied on wasn’t negatively effected by whatever the event was.
Etc, etc.
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