It's a preoccupation which extends professionally - the 49-year-old actor has often found himself offered the wrong kind of "weighty" role. He turned down Friar Tuck in Kevin Costner's Robin Hood and returned the script for a proposed BBC sci-fi series after reading his character was called "Fat Man". Nevertheless, it's as epicurian detective Henry Crabbe in the BBC's Pie in the Sky, returning to TV screens this week, that Griffiths has had his most recent success.
Despite a decade at the RSC, Griffiths is probably still best known for playing Uncle Monty in the Withnail and I. As grandiloquent host to Richard E Grant and Paul McGann, Griffiths used his size to good comic effect, memorably advancing upon the winsome McGann with the lascivious line "Do you like vegetables? I've always been fond of root crops, but I only started to grow last summer."
While Griffiths' rotundity certainly helped Monty's screen presence, it was his eloquent enunciation of "root crops" that stole the scene, a careful relish for language found in all his performances. Flat-vowelled and softly spoken, Griffiths' delicate delivery belies his bulk, scuppering expectations of bluff barnstorming. "In my imagination every shot is a close-up," he says, "I like to make it intimate, so the audience can follow whatever emotional path the character is on."
Artistic director of the Almeida Theatre, Jonathan Kent, worked with Griffiths on David Hare's acclaimed adaptation of Brecht's Life of Galileo, and admires his "capacity to make abstract ideas human and personal", and his "wit and lightness of touch".
Such sensitivity owes much to his extraordinary childhood. Born to deaf- mute parents, he was brought up in a council house in Teeside where all communication was by sign language. Until the age of five, he had spoken to nobody except the French babysitter who lived next door. "I was reading by the age of three and learned to treasure silence," says Griffiths, "and my father taught me things about body language that psychologists have been catching up with ever since. He always knew when I was lying because my posture was all wrong."
For the record, the actor puts his corpulence down to radiation he was given as a boy to treat an underactive thyroid. A stick-thin eight-year- old ballooned into a fat 11-year-old, who had a hard time at a "pretty fierce school". Girls just wanted to be his friend and the burly boy was "always branded the trouble-maker because I was the biggest".
He left at 15 with "a fist full of things called Northern Counties Technical Education Examination Certificates, which were worth the breath it takes to pronounce them. I was cannon fodder. Fitted only for non-skilled manual labour. I had to go back and requalify three years later. It drove my father nuts. He thought I should be out working."
At that time, Griffiths had never even seen a play. Which was all right since he planned to become a painter. At art school in the Sixties, however, he found that his traditional bent contrasted with the hippies around him "painting rainbows on twigs". Instead, he drifted into acting which, for his steelworker father, "reeked of poofery".
After drama school in Manchester, Griffiths joined the RSC, where he found himself frustrated in lightweight roles (his first role was as a sailor in Twelfth Night). Although his 10-year stint culminated in star performances as Volpone and Henry VIII, he felt constrained and decided to expand into new media. TV roles followed in the 1982 BBC series Bird of Prey and A Kind of Living, while on film he appeared in everything from A Private Function and Chariots of Fire to Shanghai Suprise and Naked Gun 21/2.
"For most actors there's zero choice because there's zero opportunity," says Griffiths of some of his less illustrious projects. "If you get asked to read for a movie you're competing against every other sucker in British theatre for the job. I know some people who'd throw a party if they got an audition."
One of the worst things about cinema, he says, is that "it's the most overtly hierarchical society you can ever work in. If the person higher up the pyramid is a bit of a shit, life is hell for everyone else. But everyone subscribes to it. The idea is that you hang in there, and one day you'll move up the ladder."
So much for Hollywood, what about Withnail? "Uncle Monty was great fun but difficult because I wanted to make it much more camp and [director] Bruce Robinson kept toning it down. The glorious thing about Monty is that for all his outrageousness, he's a deeply honourable man. Compared to the other scallywags in the picture, he's the best of them."
Griffiths, who returns to the Almeida this year in George Bernard Shaw's Heartbreak House, believes "theatre is the key" unless you're "one of those the camera loves". Griffiths concedes that "the camera likes me a little bit, but I must make up the rest by acting well." He describes his technique as "the swan syndrome". "To look at on the surface it's a beautiful cloud of fluffy white feathers and a haughty expression, but beneath the water there's two webbed feet paddling like crazy. You appear calm when your heart is hammering away, your mouth is dry and you're thinking `Oh God, how do I get out of this?'"
A new series of `Pie in the Sky' starts tomorrow night at 8pm on BBC1
1948: Born in Stockton-on-Tees to deaf-mute parents
1963: Leaves school at 15, hoping to become a painter
1970s/early 1980s: Stalwart at the RSC in Stratford, where he now lives with his wife, Heather
1982: Stars as a computer boffin in BBC thriller, Bird of Prey
1986: Cast as lecherous Uncle Monty in Withnail and I. `Uncle Monty stands alone,' says Griffiths of his favourite film role
1991: Appears alongside Leslie Nielsen and Priscilla Presley in the spoof Naked Gun 21/2, as wheelchair-bound conservationist Dr Meinheimer. Has a `stunt butt' to double his backside for a scene in which he has to lower his trousers.
1992: Griffiths only secures four weeks' work, `the worst feeling'. He is given 12 weeks' notice to quit his house
1994: Appears in David Hare's adaptation of Brecht's The Life of Galileo. His performance is hailed as `outstanding', `masterful' and `a triumph'. First appearance as policeman-turned-chef Henry Crabbe in BBC's Pie in the Sky. `The most frustrating thing is it's pre-watershed,' says Griffiths. `It has to be so safe.'
1997: Third series of Pie in the Sky. Begins rehearsals for George Bernard Shaw's Heartbreak House