Scott Lang: Pick on someone your own size.
Scott Lang: I didn't steal anything! I was returning something I stole.
Darren Cross: You tried to hide your suit from me, now, it's gonna blow up in your face, and destroy everyone you care about.
Scott Lang: Sorry I'm late, I was saving the world. You know how it is.
Hank Pym: Scott, I need you to be the Ant-Man.
Scott Lang: One question... Is it too late to change the name?
Scott Lang: My days of breaking into places and stealing shit are over! What do you need me to do?
Hank Pym: I want you to break in a place and steal some shit.
Scott Lang: Makes sense.
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.