Does a toy poodle stay little forever?

Yes they do, it is because of their genetics. The toy poodle was bred from miniatures, which in turn were bred from standards. Each generation, the smallest of the offspring were bred together, causing the average size of the offspring to become smaller, as the genes for above average offspring were not being passed on. Eventually when the desired size was reached, the breeding was not as tightly controlled as before, so even within the breed their is a range of sizes. However a toy poodle will never grow to the size of a miniature poodle for example, as the genes for this size have been bred out.