You can see the effect in the second animation of the linked page, but the basic idea is the lower the Reynolds number, the less likely for flows to separate and become turbulent. Shrinking the scale and slowing the airspeed both lower the Reynolds number, so paper planes have vastly different aerodynamics to full size aircraft.
And even further, insects have also vastly different aerodynamics, which explains why flapping flat wings works at insect-size and not at bird-size (nor plane-size).
