This is a raw, 1:1 copy of a physical disc. Because it is uncompressed, every WUD file is roughly 20–23 GB , regardless of how much actual data is on the disc.
There are several reasons why Wii U ROMs and ISOs have gained popularity: wii u rom iso
In the context of the Wii U, the term "ROM" is often used broadly, but the technical formats are more specific: This is a raw, 1:1 copy of a physical disc