Why are calves so hard to grow?

Short answer

It grew less than the thigh in 16 people, but grew by method. In 14 people over 12 weeks, the seated leg grew the outer and inner calf under 2%; the standing leg grew 9 to 12%. Standing raises for 6 weeks in 27 people grew nothing. Training with the heel lowered deep and doing more sets grew them more.

Why are calves so hard to grow?
'Walking all day already trains them → they won't grow': is it true?
A common saying is that calves are used all day for walking, so they barely grow with training. We checked whether calves really grew less, and which methods made them grow and which did not, in that order.
Some studies really saw no growth
Strength rose, but size stayed the same
Of 56 people, 28 did heavy calf training 24 times over 8 weeks. Strength rose, but calf girth and muscle thickness showed no difference from those who did not train. In a study where 27 people did standing calf raises for 6 weeks, muscle size did not increase either.
Less growth than the thigh in the same program
0% for the calf, 7.4% for the thigh over 16 weeks
16 people (aged 65 to 81) did calf raises and leg presses together at 80% of their heaviest lift, 3 times a week for 16 weeks. Muscle cross-section grew 5.0% in the calf and 7.4% in the thigh. The researchers of a 12-week study also described the calf as a muscle that responds poorly to training and shrinks easily when not used.
But seated raises barely grew it
Standing 9 to 12%, seated less than 2%
14 people trained for 12 weeks, one leg standing and the other seated with the knee bent at 90 degrees. The outer and inner calf muscles grew 12.4% and 9.2% in the standing leg, and 1.7% and 0.6% in the seated leg. The deeper soleus muscle grew 2.1% and 2.9%, showing no difference.
More growth with the heel lowered deep
The bottom half alone gave more than twice the full movement
42 people trained calves for 8 weeks. Doing only the bottom half, with the heel lowered, grew the inner calf 15.2%, the full movement 6.7%, and the top half 3.4%. In a 10-week study of 23 people, the leg that added bottom-range reps after it could no longer rise grew 9.6%, more than the leg that did not (6.7%).
More sets meant more growth
12 sets a week gave about 1.7 times the growth of 6
61 people trained calves 3 times a week for 6 weeks. The combined calf muscle thickness grew 12.0% with 12 sets a week and 6.9% with 6 sets. In a 10-week study of 16 people, when the total weight and reps were matched between the leg with added bottom-range reps and the regular leg, both grew 8%, with no difference.
Summary: calves grew depending on the method
Standing, lowering deep, around 12 sets a week
Seated calf training grew the outer and inner calf less than 2%, while the standing leg grew 9 to 12% in 12 weeks. In a study where 27 people did standing raises for 6 weeks, it did not grow. Training with the heel lowered deep and doing more sets grew the calf more. Standing calf raises, lowering the heel deep, at around 12 sets a week are enough.