That's true, but we're following stereotypes – the thicker the bone, the harder it is to cut through.
I liked the golem's torso in the game about Iratus – lots of skulls and protruding bones, it's immediately clear that it was assembled from several corpses, but the arms and legs are ordinary, even though they're large. I think that if there are large skeletons, they should just be resurrected, not used to build a bone construct. It's better to reinforce it with metal later.

Added 3 minutes later
I'm interested in making a golem with animal bones, attaching a deer skull, a griffin... adding more skulls. This is a standard problem with monsters – it's not clear how to improve them, and even more so, how to create an alternate upgrade.