sparse arrays - How to relate memory usage with occupied positions of SparseArrays?

What is the relation of memory usage of a SparseArray and the number of its occupied positions?

Let's say you build a $100000000 \times 10$ SparseArray and fill the two positions $(1,1)$ and $(100000000,10)$ with a value:

num = 999999;  
idSparse = SparseArray[{{1, 1} -> num, {100000000, 10} -> num}]

There are two elements in the array.

Memory usage is:

ByteCount[idSparse]  
  400000968

Disk usage is:

Export["idSparse.rsa", idSparse];  
FileByteCount["idSparse.rsa"]  
  380

Comments

mathematical optimization - Minimizing using indices, error: Part::pkspec1: The expression cannot be used as a part specification

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