More cone area. Kicker lists a 1W (or maybe it's 2.83v) rating of 82.2 dB for the 8", and 86.3 dB for the 10". That's a 4 dB increase just by going to a 10" driver -- that is huge.
So if the the 10 is 4 dB louder than the 8 with both subs running off of 1W, it will also be 4 dB louder than the 8 with both subs running off of 375W. There is so much more to look at than what % of a subs RMS power handling is being used up by an amp.
There's nothing magical that happens when you reach 'full RMS', and it doesn't really even mean you've reached the full potential of the sub (depending on the size and tuning of the enclosure used).