That's why they're called "front" vowels. With these words, your tongue depresses and folds back a bit. So "O", "A" and "U" are called "back" of the throat vowels. OK, here's the weird part.

"Front vowel" sounds such as those in bee or bay, are made with the tongue forward, while the tongue is to the rear of the mouth with "back vowel" sounds such as but or boot. "Previous work has ...

In addition, "front" vowels, like [i] again, are seen as smaller than "back" vowels, like [u], which has the same height. As Mr Pinker put it, mice are teeny and squeak, elephants are humongous ...