The above-mentioned Xenoblade Chronicles X file size is only for the Japanese version. The localized version may be slightly different due to different texts and audio files.
There are two versions of the Wii U, a basic set that comes with 8 GB of storage while the deluxe set includes 32 GB. The console does support external USB storage device, so you should be able to able to add more space with compatible drives.
The download size should come as no surprise when you considered how big of a world players will be exploring in the sequel. The game also has cut scenes and voice acting, both of which also add large amount of data.
Xenoblade Chronicles X is releasing first in Japan on the 29th of April. A North American release is being targeted for later this year.