Are bodybuilders' muscles just for show, without the strength?

Short answer

No. Muscle size was the main reason for strength, and bodybuilders' arms were stronger than weightlifters'. Force per unit of muscle size was a bit lower, and for the heaviest single lift, people who practiced heavy, low-rep sets came out ahead.

Are bodybuilders' muscles just for show, without the strength?
'Bodybuilder muscle = for show, real strength = powerlifters' — is that right?
A common line says bodybuilders have 'puffy' muscle that is big but weak. We checked how closely muscle size and strength go together, and whether bodybuilders' muscles produce strength to match their size.
Muscle size was the main reason for strength
People 60% stronger had 56% bigger muscles
16 people who had trained their legs heavily for more than 4 years were 60% stronger in the front thigh muscles than 52 people who did not train. Their muscle volume was 56% larger, and force for the same size differed by only 9%. The researchers named muscle size as the main reason for the strength gap.
Strength rose as much as muscle grew
Growth contributed more than 5 times as much as nerve changes
After 39 people trained their legs for 15 weeks, tracking each person's changes showed that how much their muscle grew and how much their strength rose went together very strongly. Muscle growth contributed more than 5 times as much to the strength gain as changes in how the nerves drive the muscle.
Bodybuilders' arms really were stronger
But force per size was lower than in weightlifters
When researchers measured arm-straightening strength in 32 bodybuilders and 20 Olympic weightlifters, the bodybuilders had both bigger muscles and more strength. But force per unit of muscle size was lower in the bodybuilders. The steeper the angle at which the fibers attach, the lower the force per size, and bigger muscles had steeper angles.
Single fibers told the same story
88% bigger and 32% more force
Researchers compared thigh muscle fibers from 12 bodybuilders, 6 power athletes and 14 non-trainers. Bodybuilders' fibers were 88% thicker than the non-trainers' and produced 32% more force. Force rose less than thickness, so force per size was 41% lower than in non-trainers.
Max weight came down to how people practiced
Muscle grew the same; max bench press did not
17 people lifted the same total load for 8 weeks, either as 3 sets of 10 reps (bodybuilding style) or 7 sets of 3 reps (powerlifting style). Arm muscle thickness did not differ between groups, but the heaviest bench press they could lift once rose more in the 3-rep group.
Powerlifters' strength also came from muscle
Bench press records tracked muscle mass most closely
Analyzing the bench press records of 13 national-level powerlifters, lean mass (muscle and everything except fat) and the size of the chest and arm muscles tracked their records most closely. Technique showed no clear link with records. Lean mass, arm proportions and shoulder strength explained 59% of the differences in records.
Bottom line: bodybuilders' muscles are strong muscles
Force per size is a bit lower, but total strength was higher
Muscle size was the main reason for strength, and bodybuilders' arms were stronger than weightlifters'. Force per size was lower, so strength did not rise fully in step with size. For the heaviest single lift, people who practiced heavy, low-rep sets came out ahead.