Because they have similar builds - including their type of arms.
Saying that Crabbe and Goyle have “gorilla arms” is basically a more creative way of saying they have very thick arms that look like they’d be good for physical violence - like actual gorillas’ arms would look. It’s logical that this description would apply equally as well to both of them - they’re similar both in build and role at Hogwarts. They’re both described as being very large and thuggish, and their role as Draco’s physical protection is made clear from the first time they’re introduced.
“Yes,’ said Harry. He was looking at the other boys. Both of them were thickset and looked extremely mean. Standing either side of the pale boy they looked like bodyguards.
‘Oh, this is Crabbe and this is Goyle,’ said the pale boy carelessly, noticing where Harry was looking.”
- Harry Potter and the Philosopher's Stone, Chapter 6 (The Journey from Platform Nine and Three-Quarters)
Their main defining characteristic throughout the series is that they’re really big, and basically serve as Draco’s bodyguards. There are very few described differences between Crabbe and Goyle - they’re usually treated as a unit.
“Loud and scathing, Draco Malfoy’s voice echoed around the courtyard. He had stopped right behind Colin, flanked, as he always was at Hogwarts, by his large and thuggish cronies, Crabbe and Goyle.”
- Harry Potter and the Chamber of Secrets, Chapter 6 (Gilderoy Lockhart)
Therefore, it makes sense that the same description would apply to both of their arms - they likely have very similar types of arms, so the description would work equally as well for both of them.