ATM Tool size does matter?

["Keir Davis" <kbd@tpg.com.au>]

.....Before I get lamb-basted for the subject title....I am referring to all
these 'plus 20 inchers' and the SMALLER tools being used/recommended.
Can some one set me straight as to why when making say a 24" mirror, a
lesser sized tool is used?

Thanks,

Keir