As the first 4 characters will correspond to one recovery word only, you will solely need to put the initial 4 characters of the recovery word on X-Seed Pro.
Users do not have to put the complete vocabulary on it.
You may refer to the this page to see the full list of the 2048 recovery words and search the first 4 characters of your own words to check: https://github.com/bitcoin/bips/blob/master/bip-0039/english.txt
Assuming there is a set of recovery words involving longer vocabulary such as attitude, bachelor, journey, etc, X-Seed Pro would be larger and heavier than the current one.
Besides, the lengths of recovery words vary between users.
Hence, considering the portability, and the feature of recovery words, X-Seed Pro is designed into the present form.