Factual error: Ant-Man is riding carpenter ants after a helicopter in flight. The ants are coming at the helicopter from under it as it rises off the helipad. This wouldn't be possible due to the down force of air from the rotors. (01:30:00)
Factual error: In the scene when The Wasp (Hank's wife) goes subatomic to deactivate a missile, you see the missile flying horizontally in the clouds. That is not possible. First, that missile does not have wings so it simply could not fly horizontally like an airplane. Second, Hank states it's a ICBM (Intercontinental Ballistic Missile). ICBMs have a circular trajectory with apogee at 1200km of altitude (750mi), WAY above the clouds.
Continuity mistake: When Scott and Hope are kissing at the end, Hope has her hands on Scott. When the shot flips to the other side, her hands are down.
Chosen answer: The suit was built human-size and then shrunk using Pym particles. The same technology can also be used to grow objects. Cross was shown to have a hand-held device earlier in the film capable of shrinking, so he either used this device, or activated the suit's built-in growing/shrinking technology.