Why does a heavier sledder go faster?

In my experience, a heavier sled rider always goes faster than a lighter one (given the same sled design). When looking at this in terms of energy, it doesn't make sense. If we disregard air resistance and friction, they should have the same KE at the bottom as given by the equations below:

ME = KE + PE

KEi + PEi = KEf + PEf

KEi = 0 at the start, PEf = 0 at the end, so we get: PEi = KEf

Or mgh = 1/2mv^2

When rearranged, the vf = sqrt(2gh)

Using these energy equations, the final velocity has nothing to do with mass... So why does a heavier sledder always go faster?

The same scenario can be applied to soapbox cars and pine derby cars. You almost always want to go heavier. Can someone explain this in terms of energy?

Thanks,

jon