OO-VEG, Don't judge anything from a take off, which you watch, unless you have a photocopy of the weight and balance sheet in your hand.
Double weight on any given aircraft doesn't mean double runway length, it is much, much worse than that.
An AN-124 with full cargo load and half full fuel tanks will probably jump in the air after cosiderably less than half the rwy roll it would need at MTOW. But then, with half full fuel tanks, the crew would almost for certain use reduced take off power. How much reduced is their decision, we can only guess. Their decision will be based upon what runway length is available.
The only thing which is pretty sure, they won't carry one ton of fuel for no reason.
Conclusion: don't look at actual take offs, look at the official figures. Anyway I assume that you are right that the AN-124 is a pretty good runway performer.
So is a B-52 bomber. At light weight it takes off almost like a helicopter. But when they were on alert to destroy the world, then their fuel load was calculated so they would exactly scrape the tail at the very end of their 13-14,000 feet long runway. And the KC-135A tankers, which would be sent on it's tail, would scrape the same runway trench even deeper.
Best regards, Preben Norholm
Always keep your number of landings equal to your number of take-offs