Typically there are 3 ways of specifying the amount of memory needed for vectors.
CASE (1) allocateNew() – here the application doesn't really specify the size of memory or value count. Each vector type has a default value count (4096) and therefore a default size (in bytes) is used in such cases.
For example, for a 4 byte fixed-width vector, we will allocate 32KB of memory for a call to allocateNew().
CASE (2) setInitialCapacity(count) followed by allocateNew() - In this case also the application doesn't specify the value count or size in allocateNew(). However, the call to setInitialCapacity() dictates the amount of memory the subsequent call to allocateNew() will allocate.
For example, we can do setInitialCapacity(1024) and the call to allocateNew() will allocate 4KB of memory for the 4 byte fixed-width vector.
CASE (3) allocateNew(count) - The application is specific about requirements.
For nullable vectors, the above calls also allocate the memory for validity vector.
The problem is that Bit Vector uses a default memory size in bytes of 4096. In other words, we allocate a vector for 4096*8 value count.
In the default case (as explained above), the vector types have a value count of 4096 so we need only 4096 bits (512 bytes) in the bit vector and not really 4096 as the size in bytes.
This happens in CASE 1 where the application depends on the default memory allocation . In such cases, the size of buffer for bit vector is 8x than actually needed.