I expected Robshaw to be heavier but that's largely because like Olyy I though O'Brien was shorter. He doesn't give the impression of 'tallness'. Anyway, not much in these stats at all.
With Tackles England and Robshaw are lucky to have so many guys tackling like maniacs... Espn stats says the following:
Parling 22 Tackles
Robshaw 21 tackle
Wood 21 tackles
Haskell + Morgan 18 tackles between them
Tom Youngs 16 tackles
Launchbury 11 tackles
Cole 10
Marler 9
I wouldn't take espnscrum stats as gospel, but it still suggests pretty great teamwork. Without looking at Irelands stats, I'd imagine O Brien and Heaslip are significantly busier than others in that area?