I'm assuming you mean that Fish do not grow to a certain size in a small tank. A fish does not grow to its tank, it keeps growing as long as you feed it, til it reaches its normal size. If theres no space, then the fish either gets deformed/sick/dies.
46Gallon Natural Planted Tank. 8 Harlequin Rasboras, 5 Amano shrimp, 2 German blue Rams, 1 Dwarf Gourami, 5 Platys, 1 Albino Bristlenose