Japanese maples will often push out flushes of new growth during the growing season, especially if they are receiving plenty of water. Depending on the cultivar, the growth can be either long, whippy shoots or dense clusters at the branch tips. The leaves generated by these growth flushes often appear quite different than the original spring growth (larger is quite normal) but after the tree loses its leaves in fall and then leafs out again next spring, everything comes back to normal.

Without providing a photo to confirm otherwise, I'd assume this is what you are seeing.

Most of my maples are pushing new growth now also. At least the ones I'm watering in pots.

Mike