A total market index fund is proportioned by market cap. As the large cap gets larger, it proportionally becomes a higher percent of the pie. If large cap is overvalued, youre owning less small cap than "true price market cap."
If, for example, you thought small caps were under valued, and you wanted to tilt small cap, you could keep doing exactly what youre doing, and just add a small investment into small caps on the side. So at Schwab, if you hold Schwab Target 2060 Index Fund (SWYNX) you can tilt by making sure Schwab U.S. Small-Cap ETF (SCHA) is 10% of your portfolio. At the moment, SCHA is 6.8% of SWYNX, so you would need an additional 3.2%. I dont know if you even need autobalancing for something so simple, just fix your allocation every time you make a contribution.
You’re trading one issue for another. By holding x% of a small cap fund, where x does not equal the actual market proportion for small cap funds, you’re choosing to over or under weight that asset class.
