so they could recycle elements in the way that was required for the cycle of life to allow future species like us to survive.

animals evolved so big back then for a few reasons, lots of free space, competition, (maybe even evolution didnt evolve the ability to have such small species, jsut like today with cellphones, they started off bigger and are getting smaller as they evolve) and i cant think of anything else at the moment but when the meteor s hit not all of them died but they did they cause they were too big and used too much energy and with all those corpses lying around the other smaller species that are around today could thrive