Wii Fit U OPENING Week Sales = 4% of Wii Fits Opening (For NA)

[Soundwave]

You're probably looking at a Nintendogs like drop off

Nintendogs (DS): 24.57 million sold

Nintendogs + cats (3DS): 3.28 million sold

Except Wii Fit U will probably sell less than Nintendogs + cats because its on a system with a much lower userbase than the 3DS.