>>6666555I have the Mafex RO Vader. I'll try to explain how the figure works and maybe even dispel some "myths" surrounding it.
He's not really that much fatter than other Vader figs but I don't blame you for thinking this since I've seen most people pose the figure carelessly. His torso is essentially free-floating and there's gaps between the mid-section and his chest to allow for a huge range of motion. That means you can't actually perfectly align the sculpt from front to back and he can only do it on one side if you shift his chest to lean completely on top of the mid-section (this mid-section is sculpted to curve inwards to allow a seamless look if he's leaning forwards/backwards). Consequently, if you pose his torso dead-center in the middle (a proper neutral stance) this means it will never seamlessly line up the sculpt on either front/back. Depending on your view this could be a negative, but it basically lends itself to a lot of range of movement, allowing him to lean towards any direction.
A common problem I tend to notice is that a lot of people always try to line up his sculpting on the front side from his mid-section up to his chest and given what I said before, you can imagine how that turns out. Pushing the mid-torso out too far in front of his chest while keeping the chest sitcompletely straight will make him look fat since you've essentially moved the chest portion too far back and made it off-center (giving the illusion of his mid-section jutting out hence the "fat" look). Beyond that, his head is actually the right size but it fits better with the imports compared to the average BS figure which tends to scale larger.
Pic-related should help.