Hi,

The short answer to using a half-inch voice coil in a 1-inch tweeter is cost.

Using a 1-inch voice coil means a larger magnet and greater power-handling capability. The magnet/voice-coil assembly is the speaker's motor--a reciprocating electro-magnetic motor--and the larger voice coil yields better high-level power handling with less distortion and improved heat dissipation. But it's also more expensive, which is why it's found on more expensive models in the Axiom lineup.

Alan Lofft,
Axiom Resident Expert (Retired)