You shrink in size with immediate huge Chakra loss. Same thing happened with Kisame and Kurama, when they got lots of their Chakra drained quickly, although Kisame and Kurama regained their size eventually, Nagato didn't until he absorbed Hachibi's Chakra for some reason. It was probably suitable for his character design.
When Nagato summoned the Mazou for offensive purposes, it was empty and it needed humongous Chakra to activate a Jutsu, so it fed on Nagato's reserves and Nagato became emaciated due to massive Chakra needed to control that big mighty statue and activate an almost invincible Jutsu. To control something as big and powerful as the Mazo with your own Chakra is impossible for pretty much any Naruto character; Nagato was obviously an exception because of his stupid Chakra reserves. Not even Obito or Madara could control the Mazo with their own Chakra and relied on Biju instead. This just shows Nagato's Chakra supply is out of the world, and the databook does say he was the candidate for the Rinnegan that Madara chose, due to his Biju reserves of Chakra.