Doubling the mass of the car assuming everything else (crumple zones etc.) stays the same - doubles the acceleration experienced by the pedestrian.

    X_p0 = X of the pedestrian before collision
    X_c0 = X of the car before collision
    X_p1 = X of the pedestrian after the collision
    X_c1 = X of the car after the collision
    m_ = mass
    v_ = velocity
    a_ = acceleration
    t = time

    //conservation of momentum:
    m_p * v_p0 + m_c * v_c0 = m_p * v_p1 + m_c * v_c1
    m_p * (v_p0 - v_p1) = m_c * (v_c1 - v_c0)
    v_p1 - v_p0 = m_c * (v_c0 - v_c1) / m_p

    // a = delta v / t
    a = (v_p1 - v_p0)/t = m_c * (v_c0 - v_c1) / (m_p * t)