リックさん、
you're right; when すぎる is used as auxiliary, it can be written in either way.
The meaning matches to the kanji, so it can be written in kanji, but auxiliary verbs are often written in hiragana
as well. It's controversial.
I personally don't agree with too many kanji writings as correct way, so I sometimes use hiragana on purpose.
There's no strict rule (or guideline) about usage of kanji, so you can see the sentence balance, I suppose.
Hope it helps!
Natsuko(奈津子),
Team JapanesePod101.com