Actually it's more complicated than that, Ashen. If the Accord were just given 'more batteries' there's still the matter of recharging them, which the car can only do through regenerative braking, and there's only so much energy that can be recovered from that. Also keep in mind the 2006 HCH, which is about 500-600 lbs lighter than the Accord, still does not have a real EV mode, because running the motor at full power would drain the battery too quickly, so it's limited to 30-40% power (or about 8 HP).

If you're wondering how the Prius gets to use 67 HP motors, or the Camry uses 140 HP motors, it's because the engine supplies the power directly to them. The maximum battery output of the Prius is 28 HP, and 40-45 HP for the Camry, but when cruising the car acts as a series hybrid, where the engine generates up to 67 or 140 HP worth of electricity for the electric motor. It sounds inefficient but it works well for technical reasons.

When the cars are in EV mode, then the motors are limited to the maximum output of the battery: 28 and 40 HP respectively. The battery does get used up but that's OK because the engine can be used to directly recharge it.

In order to compete with the TCH, Honda will simply have to give up on their inferior IMA system, but I don't see them doing that very soon.