Brian

The Atom ( ray palmer ) is incredibly powerful. His ability to shrink to the size of an atom has extensive potential.

He can get past forcefields, infiltrate mechanisms and even someone's brain ( he did it to batman ), travel across the world through a phone connection...

Plus... Antman's ability to go to molecular size & grow large was a new construct for the movies.

AM was relegated to ant size.

The Atom is an under used character.