In 1900, each person had 0.000148 of a Senator. In 2012 that is 0.00000526 of a Senator. With the new states, that goes up approximately six-fold to 0.0000316
The Constitution says there are 2 Senators per state. So, splitting California up means 10 additional seats in the senate for the people living there, so that each person's proportional representation goes up - i.e. it's easier to get the ear of your senator because you are competing with fewer people for his attention.
Not sure what would happen in the US House -- I think the number of Representatives is set at a fixed number (435) by law. So the results of the next census would be .. interesting.