It isn't the only "constant" that both big and small players must abide by. You can buy stamina with gems. It boils down to who spends money and who doesn't.
100 gems for 125 stamina, then 50 gems for 25 stamina after that isn't really a "sale" for even the big coiners eyes.
Yes, it does boil down to who spends money and who doesn't, as all things in life are. lol.