Which institution produces the largest number US PhDs in absolute terms? See here.
Science 11 July 2008: A new study has found that the most likely undergraduate alma mater for those who earned a Ph.D. in 2006 from a U.S. university was … Tsinghua University. Peking University, its neighbor in the Chinese capital, ranks second. Between 2004 and 2006, those two schools overtook the University of California, Berkeley, as the most fertile training ground for U.S. Ph.D.s (see graph). South Korea's Seoul National University occupies fourth place behind Berkeley, followed by Cornell University and the University of Michigan, Ann Arbor.
The normalized figure for Berkeley is about 6% -- six times lower than Caltech.