The difference is the location of the player information. In Daily, it is located in the right panel so there is more room for the board.
But if you click-hold the resize icon on the upper-right, you can drag left to make the board smaller, or right to make it bigger when it isn't full size.
Why am I unable to change the size of the screen during daily play. Two examples below... Is it because I'm friends with the daily player and it is random player in blitz?
e.g..
http://imgur.com/a/6I3ee