It is the instinct of magical beasts to become stronger.
This is probably the most prioritized instinct after survival.
That's what distinguishes beasts from magical beasts.
Stronger, harder, faster.
And bigger.
To achieve this, magical beasts constantly repeat their evolution.
The Silverback Chimpanzee Akims must have given up on size during the evolution process.
Something big can't climb trees, nor can it speak easily. Even its throat and vocal cords would become huge, losing the advantage of vocalization.
Also, a gigantic body consumes a lot of food as much as it is strong.
The resources of the forest are limited.
Instead, he must have wished to be fast and strong.
Riokku is different.
Was it because it was a 'bug' that was naturally small?
Was it because it was a species where weight class was clearly strength?