Osbourne told Glamour magazine Aguilera had "called me fat for years."
"I said, 'Now you're fat, too. …Now you know how that feels,'" she revealed, adding, "I'm sorry but I stand by that."
People said the fat fight apparently had its origins in Osbourne's earlier body image. She dropped about 50 pounds in 2010.
Osbourne said the trauma of being an overweight kid still stung and she wasn't going to let her mutual celebrity status with Aguilera prevent her from sticking up for herself.