Как обычно, в мануале все написано:

On 32-bit platforms, each lock requires 40 or 80 bytes of the lock list, depending on whether other locks are held on the object:

* 80 bytes are required to hold a lock on an object that has no other locks held on it
* 40 bytes are required to record a lock on an object that has an existing lock held on it.

On 64-bit platforms, each lock requires 64 or 128 bytes of the lock list, depending on whether other locks are held on the object:

* 128 bytes are required to hold a lock on an object that has no other locks held on it
* 64 bytes are required to record a lock on an object that has an existing lock held on it.