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This question already has an answer here:

Build a 100.000.000 by 10 element SparseArray.
Fill position 1/1 and 100.000.000/10 with a value 999.999.999.999.999.

num = 999999999999999;  
idSparse = SparseArray[{{1, 1} -> num, {100000000, 10} -> num}];  

Look for RAM usage.

ByteCount[idSparse] 
976 

Export it to a file

Export["idSparse.rsa", idSparse];  

See the disk usage.

FileByteCount["idSparse.rsa"]  
205000360

Is it OK like that?
Two elements only occupied. For file usage is very high compared with RAM usage.

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marked as duplicate by Oleksandr R., whuber, Yves Klett, Sjoerd C. de Vries, rm -rf May 22 '13 at 15:22

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  • $\begingroup$ I can't reproduce your observations: as one would wish and expect, the file seems to be the same (small) size no matter how large the array nominally is. I tried in version 9.0.1. Which version are you using? $\endgroup$ – Oleksandr R. May 19 '13 at 1:42
  • $\begingroup$ The two numbers 976 and 205000360 are mixed up. . .Sorry. . .So file size is small and RAM usage is large. $\endgroup$ – Hp Radojewski Schäfer Von May 20 '13 at 12:06
  • $\begingroup$ There is definitely something strange here, and the memory usage is much higher in version 9 than in previous versions. This would be a good question if you would edit it to focus on the memory usage rather than the file size. When you do so, you'll have my upvote. $\endgroup$ – Oleksandr R. May 21 '13 at 1:49

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