The Zentradi are a race made out of Giant Clone warriors. And they were the primary enemies of the first Macross series. To try to spy on the humans, they used a technology to reduce their agents down to human sized. This is usually reffered to as micronized, or micloned depending on who translates it. After the war, Humans and Zentradi made peace. Some Zentradi micronized themselves so they could become part of human society.
In Macross frontier it appears the Zentradi still prefer to fight in their original size with their own weapons, but micronize down to human size to interact with society.