the door panel (on my 04) has a plastic 6.5" ring lined with foam which is designed to press around the leading edge of the door speaker. This plastic ring isolates the front soundwaves from getting lost into the inside of the doorpanel (the area sandwhiched between the door panel and the metal skin) and by doing so forces the sound out through the grill.

A larger speaker such as a 6x8 would hit this plastic ring, and removal of the ring (or lack there of) would be detremental to your sound quality.

Bigger is not always better. Better design is better. Just put some nice components in your doors and call it a day. ok.