Why bet more on a positive count?
A positive count means more high cards than low cards remain in the deck to be played. A higher than normal chance of high cards coming out means:
The dealer is more likely to bust. He has to hit until 17, and it's harder to hit that narrow range between 17 and 21 when high cards predominate.
You're more likely to make your double-downs. When you double down on a 10 or 11, you're hoping to see a high card come out to make your hand 20 or 21.
More blackjacks. Even though half the blackjacks will go to the dealer, the "blackjack bonus" means you win 1.5 times your bet when you get blackjack, whereas you only lose 1x your bet when the dealer gets blackjack.
For all these reasons, you're more likely to win when the count is very positive, so you should bet more then. You will probably still lose the majority of the hands you play, but betting more on the winning hands helps you come out ahead.