This gene is present in a number of varieties, such as Porceleins, but is not dominant, just changes other values. At some state fairs I've see Belgians and booted Belgians that were about the size of medium sized Rock bantams. I remember seeing one Belgian Cock that stood taller than Most of the Rock Cockerals. If you do not select for smaller size your stock always increases in size for the small bantam breeds. Natural selection is for larger birds.
Size is one of the easiest things to breed for.
Between birds like Leghorns and Light Brahmas there is only around two major genes affecting body size. If you can make the first cross, in a couple of generations you can have large fowl. The problem is that lav is recessive. You know that all the progeny of the first cross carry the lav gene, but you have to breed these hybrids together to re express the gene. You can take the risk of taking the hybrid and backcrossing to the large line.