The actor told Vulture that he wanted to lose weight to play the part of bootlegger Forrest Bondurant, but his scheduling for The Dark Knight Rises prevented it.
"I wanted to be skinny," Hardy said. "Skinny, like I am now - maybe a little skinnier. But Batman came in, and then I needed to be bigger.
"I had six months to train, but there was a three-month period where Lawless was being filmed, so I had to train during Lawless. So that's why I was physically the size that I was.
"Luckily Shia [LaBeouf] was physically big as well so it kind of worked, but I would have preferred to be more Billy Bob Thornton-sized."
He went on to say that while he has gained and lost weight for roles in the past including Nicolas Winding Refn's Bronson, he plans to stop because of its health consequences.
"Shifting your weight f**ks up your liver! You've got to be careful," he explained. "It's the end of that for me."
Watch the trailer for Lawless below:
Hardy was later asked how it felt to be compared to Marlon Brando, at which point he admitted to not having seen The Godfather, On The Waterfront or A Streetcar Named Desire.
"If I am duly compared to Marlon Brando at all, well, I can only think of The Teahouse of the Shanghai Noon, that they're comparing me to that!" he joked.
"I've just got to have a sense of humour about it and not take it too seriously, because that's a big fish to be compared to. It's a very kind and lovely thing to hear, isn't it?
"That's the type of thing that my mum would go, 'Do you know what they said [about you]?' And I would go, 'Can we talk about something else, please?'"
Two new clips from Lawless were released earlier today.
> Read Digital Spy's review of the film from the Cannes Film Festival