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crushed eggs?

 An interesting paper: Why were extinct gigantic birds so small, by D.
Charles Deeming and Geoffrey F. Birchard...answers a very fundamental
question: why didn't very ;large theropods re-evolve since the K/Pg.
There seems to be no very good answer to this (I have my own ideas of
course)...but they suggest that birds must sit on their eggs and so
any mass above (arbitrary) 500kg. would crush eggs. I have a couple of
doubts about this: 1. I'm not sure that birds actually do _sit_ on
eggs. Don't they rather create a warm environment and support their
bodies over the eggs? 2. The authors suggest that large non-avian
theropods created nests in leaf mounds to get around this
problem...but they seem unaware that birds (megapodes for e.g.,) can
do the same. 3. Rather amusingly, the authors suggest that the
solution that ratites have arrived at...paternal incubation...allows
for a larger female and therefore a larger, stronger egg, but that
this necessitates a smaller male so as not to crush the eggs...then,
because of copulatory imperatives, the male cannot be that much
smaller. I think that this rather under-estimates the determination of
males on the make!
I would appreciate any comments.