I think the main issue is sail area -

Spitfire http://www.catamaran.ie/cats/spitfire.htm
Main 15.5sq m
Jib 4.50 sq m
Spin 18 sq m

Formula 16 http://www.formula16.net/content/view/18/40/lang,en/
Main 15.0 sq m
Jib 3.70 sq m
Spin 17.5 sq m