Technically it is not one word but two: 大きい + 過ぎる. The latter is called an auxiliary verb, and they can be combined with other words, like you see in this example. In Jisho, entry #6 explains its usage:
This is a super-common phenomenon, so it is worth knowing about it.
As to why it is not in WK, please keep in mind that the vocab words are mainly there to reinforce the kanji readings, not to teach vocab as such. The readings are already covered with WaniKani / Vocabulary / 大きい and WaniKani / Vocabulary / 過ぎる .