I find that boat weight does seem to make a significant difference in its "twitchiness". For example, I used a 15 kg carbon millenium for a few years and sat on about 3 cm of padding, sometimes more, and it was fine stability-wise. Then I switched to a 11 kg millenium (vac bagged of course), and it felt horribly unstable with only about 1 cm of padding to sit on. I had chalked it all up to the weight difference, but ski deflection is a good idea as well. Though, my guess is that the 15 kg millenium would be stiffer as it was like a rock.
Interestingly, I demo'd a V10 L Ultra last fall on flatwater. Hopped from my 15 kg millenium and 4 cm of padding, to the Ultra. My millenium was much less twitchy. Again, I chalked that up to boat weight and not stiffness.
I've paddled an 11 kg Mako 6 and a glass 16 kg Mako 6, and I haven't noticed too much difference in feel, though the lighter 6 is a bit , and only a bit, twitchier. But, I would guess that both layups deflect by the same amount as the lighter one is not rock-solid-stiff. Great ski, though.
