If memory serves, they were never explicitly stated to have large chakra reserves. They were said to have powerful life forces, strong sealing techniques, and Kushina was said to have a unique chakra, and Naruto, stated by Yamato, has powerful chakra. It seems they have powerful chakra and above average reserves.
I can't count Nagato or Naruto, as they have been influenced, by Kurama and the Rinnegan and therefore can't really be used as a measure for the whole clan. The only pure Uzumaki we really have to judge the whole clan by is Karin and that doesn't really seem fare to do.