Land animals weighing more a couple Kg were wiped out. Small critters tended to survive, they were more numerous, and widely dispersed. They also tend to be hibernators, that helped. They need less food, etc.
The classic bigger they are, the harder they fall.
On land roots and seeds tend to survive fire and soil tends to buffer acids so vegetation could re-establish fairly quickly.
In the ocean, low pH precipitation, 'nuclear winter' darkness and surface heat kicked the legs out from under the food chain. Plankton doesn't go to seed, or have roots...
Yes, and with the reproductive rates of insects and other small animals the main limitation on speed of bounce-back is the recovery of the food plants.