A big part of rugby is winning the collisions.
When you've got a lot of little guys in the team, you start to lose too many and too many in a row - and you get hurt.
You need a few playmakers - the best of whom tend to be a bit smaller and you have to accept they'll get flattened or run over occasionally - but, I like to have plenty of big guys around them so that there's bail out options regularly on hand and guys who can cover defensively. I miss Ma'a Nonu!
When we start fielding an under-sized 8 in Ardie, Aaron Smith, Mo'unga, Havili (a smallish 12), Sevu Reece and DMac that's starting to get a bit Japan-like to me.
I'm inclined to throw in a few more bigger bodies.