The largest species were those that were most affected (large dinosaurs were the most affected), while the smaller ones were better adapted for survival. First, not all dinosaurs died out. Modern birds are descendants of ancient avian dinosaurs, which also makes them dinosaurs according to the phylogenetic nomenclature of the species.
Secondly, humans did not exist at that time and the oldest human fossil (Homo sapiens) we have is 300,000 years old. This means that there are about 65 to 66 million years between the K-T event and humans. (Edit: this is "only" 63-64 million years old if you count the entire Homo genus)
In fact, this extinction event has released many ecological niches for mammals to occupy and thrive. Without the K-T event, it is likely that humans would not have appeared.
I know you explain this is useless, but deep inside me, I always have the hope that people like you are actually learning something.