There are more Democrats, because of:
Young People (18-25) - Only 10-15% of these people usually vote. This is true even with increases of them registering to vote.
Operation Chaos - During the nomination process, after it was clear McCain would be the Republican nominee, Rush Limbaugh encouraged Republicans to reregister as Democrats so they could vote for Hillary Clinton in the Democratic primaries. This caused the DNC nomination process to be dragged out.
While there are technically more Democrats than Republicans, this difference is really simply an illusion.
Now does this artificially inflate the expected PUMA factor?