German giants were selectively bred to bring out size and vigor in bearded dragons more than color or pattern originally. Developed in Germany, hence the name. Since then other breeders have added that line into their own lines to add color and other features to the German giants.
That is pretty much the basics on the line. Others may be able to tell more details. Though I don't think too many breeders are still trying to maintain the original purpose of German giants, the breeding for large and vigorous offspring. Seen quite a few small 'German giant' crosses...though the more pure GG's are quite large still.
-----
PHLdyPayne