When the number ofvectors too big, index build will fail to complete!

When there are too many vectors, such as 5 million （n = 1024×1024×5）， index build will fail to complete.
The program stops at this place, prompting:
"Save Data To xxxxx\vectors.bin"
And I noticed that the file size of vectors.bin reach to 300G! that is not normal，the The correct file size should be 10G.

The code that caused the error is as follows：

import SPTAG
import numpy as np
n = 1024*1024*5 #this szie will cause the file size of vectors.bin to reach 300G!
k = 3
r = 3
Dimension=512 #the size of vectors.bin will be 1024×1024×512×4=2G  n×Dimension×4
def testBuild(algo, distmethod, x, out):
   i = SPTAG.AnnIndex(algo, 'Float', x.shape[1])
   i.SetBuildParam("NumberOfThreads", '4')
   i.SetBuildParam("DistCalcMethod", distmethod)
   ret = i.Build(x.tobytes(), x.shape[0])
   i.Save(out)
def Test(algo, distmethod):
   x = np.ones((n, Dimension), dtype=np.float32) * np.reshape(np.arange(n, dtype=np.float32), (n, 1))
   q = np.ones((r, Dimension), dtype=np.float32) * np.reshape(np.arange(r, dtype=np.float32), (r, 1)) * 2
   print ("Build.............................")
   testBuild(algo, distmethod, x, 'testindices')

if __name__ == '__main__':
   Test('BKT', 'L2')

How can I solve this error?
Please Help Me！