Does it work that way?
I doubt it.
I mean, if you started a 120 newbie on 3 grams of gear, he would grow. And he would continue to grow. He would continue to grow until his body got to the maximum size that 3 grams would support. It would take a long time.
He would not grow from 120 pounds up to 140 pounds, and all of a sudden he would need 4 grams to get to only 150 pounds of bodyweight. It does not work like that.
Would 3 grams be too much for this newbie 120 pounder? Yes, obviously, but not for the reasons you state. It is too much because it comes with all the side effects from 3 grams of gear.
The newb could continue using 3 grams until he finally reached the same body weight that 3 grams would support on his frame. This is going to be the same final body weight as if his first cycle was half a gram of testosterone weekly for 10 weeks and he worked his way up to 3 grams. It would be much healthier to start out with the least amount of gear to get to the next goal, and it would be more comfortable. But the 120 pound newbie would not get stuck at some lower bodyweight and need 5 grams just because he started out with "too much."