How much more muscle do bodybuilders have than regular people?

Short answer

Drug-free bodybuilders carried about 10 kg (16%) more lean mass than people who don't lift, and their upper-arm and back muscles were 1.6 to 1.8 times larger. The fibers got thicker; their number did not change.

How much more muscle do bodybuilders have than regular people?
'A few years and your muscle doubles' — does that hold?
On stage, bodybuilders look like they carry twice the muscle of a regular person. We checked how big the gap really is, for the whole body and muscle by muscle, using studies that scanned inside the body.
Whole body: about a 10 kg gap
Body weight differed by 5 kg, lean mass by 10 kg
A study compared 15 drug-free bodybuilders, who had trained 6 days a week for 11 years on average, with 15 people who don't lift. Lean mass (muscle and everything except fat) was 72.7 kg vs 62.8 kg, about 10 kg (16%) more. Body weight, 86.1 kg vs 80.9 kg, was not clearly different; fat was lower, 10.0 kg vs 14.7 kg.
Muscle by muscle, the gap was bigger
Thighs 1.3 times, upper arms and back 1.6 to 1.8 times
The same study measured muscle volume by region. Thigh muscle was 5.8 L vs 4.5 L, 29% more. The front upper-arm muscle (biceps) was 64% larger and the broad back muscle (lats) 83% larger. The gap was biggest in the upper body.
Muscles grew by getting thicker
Fiber count was no different from regular people
Comparing upper-arm muscle in 12 bodybuilders and 13 regular people, muscle size differed a lot but fiber number did not. In thigh samples from 12 bodybuilders and 14 regular people, a single bodybuilder fiber was 88% thicker.
Where is the drug-free limit?
About 81 kg of lean mass at 180 cm tall
On a height-adjusted lean mass index, the median for young men among 5,635 Swiss adults was 18.9, about 61 kg of lean mass at 180 cm (5 ft 11 in). Among 74 men who never used steroids, nobody passed 25.0 (about 81 kg). The 20 Mr. America winners from the pre-steroid era averaged 25.4.
Drugs blow past that line
Same training, 15 kg more lean mass
15 steroid-using bodybuilders trained for as many years and days as the drug-free group, yet had 87.8 kg of lean mass, 15 kg more. Their heart (1.20 L vs 0.89 L) and liver (2.57 L vs 1.82 L) were larger too. Drug-free bodybuilders' organs were no different from regular people's. In a survey of 157 men, some steroid users passed an index of 30.
Bottom line: the drug-free gap is '10 years, 10 kg'
The 'double' from the first card showed up in no muscle
Drug-free bodybuilders who had trained for 11 years on average carried about 10 kg more lean mass than regular people. By region that was 1.3 times for thighs and 1.6 to 1.8 times for upper arms and back. Fiber number stayed the same; fibers got thicker. On contest day, 11 drug-free competitors scored 22.8 to 23.8 on the index, inside 25.