big vs. small
I think in this discussion we are talking about two boats of equal displacement but different lengths. Almost by definition the longer boat will have a proportionately narrower hull and will spread its displacement over a longer length. Similarly the center of buoyancy would tend to be higher. It also means that potentially, the vertical center of gravity would be lower. This means that the proportionately longer narrower boat would be less prone to pitching and rolling and would develop a higher ballast stability than the wider shorter boat. Additionally the more easily driven hull of the longer boat requires less sail area to make good progress in heavier going and can disburse more of the energy of gusts through acceleration rather than heeling and so would have more apparent stability.