Yahoo is about 40% the size of Google, but if you don’t have parallel good rankings on the two the comparison doesn’t really mean anything. Remember that “yahoo” right now does also mean MSN, which by my estimates is about half the size of Yahoo, so together they are about 60% of Google. They both deliver good traffic, but depending on the terms you may or may not have Overture ads above you.
“Less than expected” is not a very distinct term… if you expected the moon, well it will be less. Personally Yahoo has always been about what I expected, about half of Google.
Perhaps you are really wondering if the traffic you get from certain terms is less than you expected.