Larger subs require more internal airspace in the enclosure... Physically you might fit a larger sub, but you will not have the airspace to have any type of low end extension (low bass). We use two 6.5-inch subwoofers in a high-efficiency vented enclosure and based on what we see in the market, the only one that fits and allows full functionality of the rear seat. If you are willing to lose functionality of the rear seat, you gain airspace and larger subs can happen due to the gain of airspace.
Space makes bass, not the size of the woofer alone.