I think “yao” shows comparisons under this circumstance, and can be seen as abbreviation for “还要/hai yao”.

eg：

-你比我想象中还要美。You are prettier than I thought.

-你做的饺子比餐馆做的还要好吃。The dumplings you made is better than those of the restaurants.

“要/yao” is quite colloquial, and normally used with “要......得多”

eg：

-去地铁站的距离要近的多。The distance to subway is much closer.

-这部电影要有趣的多。this movie is much more interesting.

要 is used in comparisons to indicate an estimate:

树荫底下比屋子里要凉快些。 

shùyīn dǐxia bǐ wūzi lǐ yào liángkuai xiē.

It is cooler under the tree than in the room.

Source: Pleco

It’s not translated into English.