I agree with petegreg about it being juvenile foliage, but it isn't ALWAYS caused by a tree being stressed. It can also be triggered by a tree that is really healthy. For example, if a tree is root-bound and you repot it in good fresh soil, you may get some juvenile foliage in the short term. You can also sometimes trigger it by fertilizing.
However once the growth rate slows, new growth will revert to scale foliage.