The theory isn’t completely dead, It could still apply to critters without a decent circulatory system and set of lungs/gills.
Dragonflies with 3 foot wingspans come to mind.
(Side question, how much atmosphere did the Chicxulub impact blast into space?)
No, the theory says the SAME animals that live today were larger in the past.
Haven’t found ancient blue whale fossils from that period yet, but new finds are happening all the time.....
Most marine animals would not be killed in a meteor strike/flood due to the buffering effects of living in a deep ocean....they might die later of starvation, but that is a slower event that preditation can destroy evidence easily...
In a functioning eco-system, nothing is left to waste, there are very few animals that are not fully consumed after death.