Hope I can get my point across
For muscles to grow they need more stimulation then they are used to.
For sake of arguement lets use someone who has never trained before.
He gets to the gym and wants a big chest so he does flat bench press. Starts out with 100lbs and does that same weight for 2 months and gets some growth.
subject 2 new at training same size strengh starts at 100lbs but keeps increasing his weights and in 2 months reaches 200lbs.
Will subject 2 be any bigger? My point is muscles only grow at a certain rate