Just looked at Chicago. The other major cities don't have complete counts yet, so I can't really compare. As of right now, Cook county (Chicago), has 200k less votes for Biden than Clinton, but I expect those numbers to go up. I don't believe you'll get anywhere near a 25% increase though, at this rate.

Edit: If I take the remaining votes in Chicago, and give them to Joe at the same percentage he's currently at in Cook (73%), The best he can increase from 2016 is 9%, which is inline with this year's election numbers. These 46% jumps are only found in these swing state counties. They are abnormal when you compare them to other blue cities that weren't in play. It's very difficult to increase turn-out by that much. Not impossible, but very unlikely.

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