Most people are the same size forever and ever because they use the same shitty routine that doesn't work with the same shitty form that doesn't work with the same shitty eating habbits that don't work day after day, week after week, year after year.
Most people come to the gym on monday to max on bench with their asses a foot off the bench, and pump up their chest and bis, they come in on tues to do lat pulldowns with the entire stack, not knowing how much it is, while they swing their bodies as if having a seizure and still only manage to move the bar in a 2 inch range of motion, they then do some type of row tht looks like a pelvic gyration, then they move on to every type of bicep curl inaginable with the worst form one can dream of.....Wed is a day off from all the "intense" training....then they come in on thurs to do 1/16th of a rep milirary presses on the smith machine, swinging side raises with more weight than they have any business touching, 1/40,000th of a rep shrugs with 8 plates a side, and some partial ROM skull crushers with 100% assistance from a spotter. They spend thurs night - sun afternoon pouring alcohol down their throats, eat about 1,000 calories a day and do this for years.
Thats why the average gym member, despite being considered "built" or "buff" by the average person who doesn't workout never seems to grow or progress despite being a member of a gym for 5 plus years....lol.....I noticed this phenomenon myself.