Davis is one of the NFL's biggest men today because his parents, both of whom had lost spouses, found each other in small-town Texas and decided to add one more child to their already enormous blended family.
He became No. 22 in the brood, between them.
No wonder Davis has been such a perfect fit in Jim Harbaugh's family-oriented San Francisco locker room since the 12th-year guard signed with the 49ers this summer.
When his mother remarried, Sammie Lee Davis never doubted she would have one more child to give her 11.
Little did she know, her last would wind up a 6-foot-6, 355-pound offensive lineman. After all, Davis was born at a healthy, yet rather average 7½ pounds, less than a month after she turned 40.
He got big in a hurry.
''I've been the biggest since Day 1,'' Davis said, ''From kindergarten all the way through.''
In fact, Davis began growing so fast he was out of his infant clothes in no time, and he fussed so regularly his mom finally realized he was just plain hungry. All the time.
''I started to put a teaspoon of baby Rice in his bottle with formula, and it was just like you gave him a sleeping pill,'' she recalled.
Sammie Lee was twice a widow when she married Davis' dad. She lost her first husband in a car accident and her second to cancer. L.A. Davis, a minister who died in 2007, was already a father to 11 of his own before his first wife died of a hemorrhage.
One of his young sons was in school with one of her daughters.
Davis is doing a much better job keeping track of the 49ers' complex playbook than he ever has remembering how many nieces and nephews he has, let alone their names.
''People don't have that many kids nowadays,'' said Davis, a father of two girls, 9-year-old Meeya and 7-year-old Mariya.
Davis emerged as a rock-solid run-blocker during his best seasons from 2007-2010 with the Dallas Cowboys, who released him before training camp in 2011. He started every game from 2006, his last season in Arizona, through 2010.
Yet the 34-year-old Davis didn't play a single snap last season after signing with Detroit in November.
Then, Davis had offseason foot surgery. Harbaugh gave him a shot after Davis visited the 49ers this spring. He signed four months later, ahead of training camp.
His former Lions (1-0) come to town for San Francisco's home opener Sunday night at Candlestick Park in a matchup of projected NFC contenders. Davis comes in as an extra blocker, lining up at tight end, in some sets for the 49ers (1-0).
''It feels like he's fit in, he's one of us. Another guy that's just a pure joy to be around,'' Harbaugh said Monday. ''Pure, pure athlete and good guy.''
Harbaugh was impressed by Davis' technique and fundamentals in a new system from Day 1 – and Davis taking it upon himself to spend extra time with offensive line coach Tim Drevno to learn.
''It speaks volumes for him, his ability to be able to come in and get up to speed this fast,'' Harbaugh said.
A three-time Pro Bowler, Davis said all the right things for a player coached by Harbaugh when he arrived. Namely: He wouldn't be here if he didn't like to compete.
''Leonard's been a great addition,'' linebacker Patrick Willis said. ''He's a big guy and we know he still has some oomph to him.''
When an IndyCar was displayed at a recent practice, Harbaugh joked that Davis probably weighed a third of the car.
''Big Leonard,'' as he is sometimes referred, didn't mind, knowing it was all in good fun. If Davis was sensitive to jokes about his size, he might have walked away years ago.
He earned the nickname ''BIGG'' after reaching 6-0, 205 pounds as a fifth-grader. By seventh grade, he was 6-4, 300.
''I've been big all my life,'' said Davis, drafted out of Texas by the Cardinals with the second overall pick in 2001 behind Michael Vick. ''It's not like I graduated high school 200 pounds and three years later I'm 350 or whatever. It wasn't like that. That kind of stuff doesn't bother me. Plus, for Coach to say something like that, he's not saying it in a mean way. That's just all jokes.''
Most of his siblings had already moved out of the house by the time he came along, though he grew up with three sisters and a niece and nephew in the house. And the sister closest to him in age was 10 years older.
''It was stressful at times. They did whatever they could do to keep the family moving along,'' said Davis. ''Sometimes you have heartaches and sometimes you have joy. Most of it was joyful living.''
His mom did all she could for Davis, who grew up in the close-knit, sports-crazy community of Wortham, Texas, with a population of just under 1,100. When the football team traveled, the town traveled.
Sammie Lee traveled, too. She once drove 50 miles to find three pairs of size 18 Nikes that were on clearance at $19.95 because the person who ordered them never showed up – and there wasn't exactly a high demand for the size.
''They were blue and white. They didn't match his college colors but Leonard was so happy,'' his mom recalled. ''His first pair of dress shoes, my brother found them and my daughter and I drove 91 miles to get those after they were held for us. ...
''I tell people I could write a story on how we raised Leonard. I don't regret anything,'' she said. ''I've been to more places. Leonard built us a home, bought us a new Tahoe car when he got picked in the NFL because my husband was tall and it was hard for him to get in and out of the car. Leonard, he took care of us. In Austin where he went to college, they called him the big teddy bear.''
Davis graduated high school at nearly his current size. He is happy to say he was rarely teased for his unique size.
''Where I'm from, everybody was friends, everybody got along,'' he said. ''We all knew each other, we all respected each other. It was pretty family oriented.''
These days, Davis is happy with his brothers on the 49ers. San Francisco won 30-22 at Green Bay in its opener last week, and the defending NFC West champions are quickly showing they're serious about making another run at a Super Bowl after last season's near-miss.
''There's definitely a lot of talent on this team,'' Davis said. ''The guys who are here want to be here.''