I'm not an accountant, so I can't say, but it looks like you're right and Im not sure why. For AMKR, EBITDA/rev = 0.24, for ASX, EBITDA/rev = 0.30. So for a given amount of revenue, ASX seems to be more profitable, before the interest etc, is taken out. Which makes AMKR cash operating expenses proportionately a lot higher than ASX.
Maybe AMKR is more intensive in higher tech packages, whereas ASX has a higher proportion of memory packaging business, which has picked up in the past year, and is less expensive from an operational point of view. Just a guess. Although the higher tech packages will carry a higher profit margin, maybe ASX is just simply doing a booming business in memory.
It's the "I" in EBITDA. Or better said, (one more time) it's the debt! But as Kim gets $15.5 million of the total interest payment every year, it isn't going to change too quickly. But outside of the CBs they are doing a good job on paying down debt and reducing the "I."