If the same person can gain 15lbs. from 250mg weekly and 20 from 500mg weekly:
100% increase in dosage yields 25% increase in gains - this is diminishing returns where each successive unit of weight gain demands a greater increasing dosage to the eventual point of total saturation. (numbers are out of my head but probably close to what guys are getting)
The trick is to find the balance. Personally, everybody gets gains with 500mg if they don't they have a major problem. Only the people who have good receptors or really know how to train gain well on less.
I can't remember meeting anyone in the gyms recently that I believe knows how to train correctly. That includes a fair amount of roido boys. Before the internet, no one would hook the newbie lifter up with juice - why bother when the guy doesn't even know how to lift correctly. Now anyone can get it. I think the increased dosages are a reflection of this factor as well as the general excess in the sport of bodybuilding which is a sport (loose interpetation - maybe more of a beauty pageant as the similarities are striking) based on increasing drug dosages.
In the 50's - everybody gained of 10-15mg of dbol. This is what the guys in venice agreed to limit their dosages to as well as what the lifters in York started with. Now every first cycle needs 30mg as a kickstart.
Go figure...excuse my rant. Somday you guys will be explaining to people that no one needs 50mg of dbol and 1000 mgs of test for a first cycle.