Option 1: Ant Rush that overpowers Marisa and eat her alive Option 2: Notoriously big Soldier Ants Killing Marisa for threatening a worker Option 3: Kirishima Logic Kicks In Option 4: A Mister Human that sympathizes with Ants punishes the uneasy Marisa and then gives lots of sweet-sweets (read yu filling) to the ant.
Newborn koyukkuri are generally considered to be around ping-pong ball sized, so actually, that ant may be around 'medium' size as far as ant species go....