With 16ga shooters, especially those new to the gauge, it isn't so much an obsession to shoot heavy loads in a light gun as it is the nonavailability (other than via UPS, or at an unusually well-stocked gunshop) of anything lighter than the "promo" 1 oz loads from Win/Fed/Rem--which are not all that light, because American ammo makers turn them out hot enough to make autoloaders cycle.
As pointed out above, it's not the length of the shell that causes problems--it's the service pressure to which the shell is loaded. In the case of a French double in good shape, however, pressure isn't likely to be an issue as far as damaging the gun. But the American "promo" loads will really punish the shooter when run through a light 16.
