-Removed-That measurement is only the engine.
The frame required to carry around that engine is not "negligible".
600ccs is 50% bigger than 400ccs. The weight difference could be 50% more. There's a big difference between 300 lbs and 450 lbs. Often the larger engine requires different materials, like a heavier frame material, making the curb weight difference possibly substantial. Age of the bike is another factor. Newer bikes use newer, lighter, composite materials. The weight difference between what can be dragged off the road, versus what can't, could be critical if that difference surpasses your ability to move it, even if it's just 20 lbs, and as I've already stated, the difference could be more like 150 lbs.
Actually, I just checked. (And you accuse me of not doing research.) I was overstating the difference, but it's all about where that difference lies. My 400 has a stated curb weight of 366 lbs, which is very near my limit, but the 600 I was looking at has a weight of 412. Yeah, only 60 lbs, but as I was saying one I can do, the other I cannot. I don't expect to be laying it down often, though, let alone mangling it. Just picking the thing upright is a lot easier, as they are low center of gravity. The clearance on mine is only 5.5 in. What stopped me getting the 600 was exactly that, I couldn't easily push it around, even upright. The acceleration was scary, so I ended up stalling it out at one point by going too slowly. Just not manageable. The 400 is way easier, at least for me.
Last, but not least, the 600 was nearly twice as expensive, even comparing last year's model to the 400's this year's model.