It's not the hood that makes the "big fat wide comfortable shape" -- it's the lever body underneath. The hood is just a thin rubber covering. Thus, Shimano brifter hoods can't be used on older brake levers. The lever body shape is very different, as you've observed.

If you want a wider, more ergonomic feel on your vintage Cannondale, your best bet is to find some comfy levers, not just replacement hoods. I like the feel of Tektro's RL340 brake levers, which should be a straightforward replacement for your Shimano 105 levers. They can be found in the $25 range online. There's an XLC-branded version of the Tektro levers that can be found for even less -- currently $13.04 from Niagara Cycle.