From what I understand, Yuji wasnt at the X-Games.
Im not so sure about Yamaha needing a bigger rider. Look at how well Taylor is doing compared to how well Malinoski did. Taylor seems to have better overall finishes than Malinoski did.
Also, the FX Nytro really isnt all that much heavier than the 2-strokes. Its something like 10-20 pounds heavier, but thats it.
If you go on the Yamaha blog, one of the posts about Duluth makes the comment that the Nytro isnt the lightest snocross sled, but it isnt the heaviest either.