Did you not read it?

30 million years after the asteroid event there were mammals 3 times the size of Tyrannosaurus Rex roaming the land.

All sorts of factors go into animal sizes. Oxygen will be one of them.

Contrary to your OP, land based mammals grew significantly for millions of years following the asteroid event because suddenly they weren't competing for food and space with as many other creatures.

http://phys.org/news/2010-11-size-mammals-dinosaur-extinction.html

Oxygen levels also affect sea life though - just because there's a beach in between us doesn't mean they live in a different ecosystem. Their oxygen still comes from photosynthesis. It still needs sunshine. If oxygen levels have led to shrinkage on land, it should have followed in the sea - but actually, oxygen doesn't seem to have such a huge effect as gravity or competition for food.

Isolation can also play a part, see examples of Island Gigantism (http://en.wikipedia.org/wiki/Island_gigantism - likely due to decreased predation) but also Insular Dwarfism leading to the opposite effect (http://en.wikipedia.org/wiki/Insular_dwarfism - likely due to increased competition for food and space).

There are so many factors that go into 'deciding' a species' size that it's never just one thing.