John Cena Says Shaved Head Means More Hair After Second Transplant

John Cena has a big head ... full of new hair after his second transplant -- and he revealed he spent nearly a day under the knife combined for both his surgeries.

The retired pro wrestler and actor combed through smart-ass jokes and jabs after he shaved his head for his second hair transplant ... telling PEOPLE there was a method to the cueball madness.

Cena says he shaved his head so his mane doc Ken Anderson could use the follicles for his second transplant -- which he did 15 months after his first procedure when he noticed an area he wanted fuller.

The film and TV star spent nearly a day in surgery between the two transplants ... and haters be damned because he's full of confidence with his new 'do ... and zero shame.

There's probably a hair today, gone tomorrow joke in all this ... but John's a big dude, so we'll skip it.