Mayra Rosales has shed 800 pounds since she shot to fame for allegedly killing her son with her obese body (Nightline)

Mayra Rosales, the "Half-Tonne Killer", has revealed that she took the blame for a murder her sister committed because her huge waistline made her feel like she was going to die anyway.

Rosales, 33, rose to prominence in 2008 when she was accused of crushing her nephew to death with her once 1000-pound body.

However, it was her sister Jamie who had in fact committed the crime and who is now serving 15 years in prison.

"I tried to protect my sister, and I was already dying so I didn't see ... like I was doing (something) wrong," Rosales told American television channel ABC.

"I was already giving up in life, and the way I saw it, when they began saying I was going to get capital murder charges, I really saw it like they were doing me a favour."

"I knew I was not doing right, but I was willing to give up my life for her," she continued.

Now, the Texan, who used to be one of the world's largest women, has dropped to 200 pounds and her name has been cleared.

"I have lost over 800 pounds," Rosales said. "I don't even know how I survived that. It's a miracle of God. I really was dying."

"I used to take up a whole king-size bed," she said. "I couldn't even stand up on my feet. My legs couldn't hold my weight."

To lose 800 pounds, Rosales committed herself to a strict protein diet and underwent several surgeries. She is now in the process of trying to adopt her jailed sister's children.

Rosales with her partner revealing the dramatic weight loss since 2008 (Facebook)