[null and void]

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?)

[EnderWiggins]

"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.

A 30 inch wingspan is not the key measure. The body of the Meganeura "dragonfly" was only 18 inches long. There are walking sticks that are 13 inches long that are living today. So... yeah they were big, but not uniquely so.

The real problem though is that gigantism doesn't actually associate with specific time frames. Different organisms or types of organisms express gigantism at different times. They don't provide a pattern of generally larger to smaller over time, or as the result of a certain event where suddenly organisms generally became smaller.

Certainly, the fossil record tends to preserve larger organisms preferentially over smaller... and we also tend to emphasize in our media the extremes over the ordinary. Giant dragonflies and cockroaches of the Carboniferous get our attention... but the simple truth is that most dragonflies and cockroaches of the Carboniferous were little bitty guys, just like today.

As to your aside... it is not clear to me that the K-T event would have blasted any atmosphere into space. I certainly cannot find any references or models that would indicate it did. (Side question, how much atmosphere did the Chicxulub impact blast into space?)