It's not a question of proportion, though I'm guessing that wasn't the word you were going for there.
Before we can address the reason the carry trade is linked in with other markets we need to look at how the positions are put together. It's not just a question of shorting Yen and converting them in to Euros or Sterling or Aussie dollars. The big players are borrowing the Yen in some fashion, shorting Yen assets or issuing debt of some kind. They are then taking that money and putting it to work in other markets, after converting it obviously. If something happens in either the market for the Yen part of the position (the short or debt) or on the other side of the equation where they are holding the other assets (or whatever) then it impacts the value and/or return of their positions. That potentially ties in with all kinds of other stuff they might be holding.
The bottom line is it's all inter-connected. The irony of trading houses and funds becoming more sophisticated in managing their portfolios is that they have created linkages between markets that otherwise would not have been as linked because they have sought to create non-correllated positions. That's why when things thing start to go bad, they can really, really go bad.