The conversion from csv to binary format reduces t

I have csv dataset of size 5.2GB ( Taken frm here). It has about 7M rows of dimension= 29. The values are of type float64. I want to convert this dataset into a binary file. To do so, I do the following simple lines:

import numpy as np
import pandas as pd

df = pd.read_csv('data.csv', sep=',')
np.asarray(df.values).tofile('data_binary.dat')

A snapshot of the data looks like this:

0.000000000000000000e+00,9.439358860254287720e-02,1.275558676570653915e-02,9.119330644607543945e-01,-9.083136916160583496e-02,-2.335745543241500854e-01,-1.054220795631408691e+00,-9.759366512298583984e-01,-1.067278265953063965e+00,-6.138502955436706543e-01,7.542607188224792480e-01,-9.256605505943298340e-01,-5.289512276649475098e-01,1.235263347625732422e+00,8.606486320495605469e-01,-2.320102453231811523e-01,-4.043335020542144775e-01,-1.559396624565124512e+00,-8.154401183128356934e-01,-1.376865267753601074e+00,6.759096682071685791e-02,1.372575879096984863e+00,-5.736824870109558105e-01,-1.368692040443420410e+00,-4.793794453144073486e-01,1.529256343841552734e+00,-5.757816433906555176e-01,-1.290232419967651367e+00,4.999999694824218750e+02
1.000000000000000000e+00,3.272003531455993652e-01,-2.395536154508590698e-01,-1.592038273811340332e+00,-2.324983835220336914e+00,-5.070934891700744629e-01,1.574625492095947266e+00,-1.050106048583984375e+00,9.686639308929443359e-01,1.312386870384216309e+00,7.542607188224792480e-01,-9.113077521324157715e-01,-1.718587398529052734e+00,3.751282095909118652e-01,8.606486320495605469e-01,-3.711451292037963867e-01,-5.625200271606445312e-01,-2.721544206142425537e-01,-8.154401183128356934e-01,-3.339428007602691650e-01,1.058411240577697754e+00,4.364815354347229004e-01,-5.736824870109558105e-01,-2.172690257430076599e-02,-5.791836977005004883e-01,-3.260441124439239502e-01,-2.024624943733215332e-01,-4.585579931735992432e-01,7.500000000000000000e+02

The new binary file data_binary.dat is reduced to 1.5GB. This is huge reduction which made me wonder if something went wrong with the way I use to convert csv to binary format. Is this reduction expected? At least this much? Thanks

Ok, so I went and downloaded a sample of the data. Each row is something like:

0.000000000000000000e+00,9.439358860254287720e-02,1.275558676570653915e-02 ...

Each individual number seems to have 25 character overall, and actually, 26 or so if you include the comma. So that's one byte per character, so about 25 bytes. Using a binary representation of a 64-bit floating point numbers will require ... 64 bits i.e. 8 bytes per number. So You should expect the binary file to be less than 1/3 the size, so this seems correct:

5.2/3 = 1.73...

A better estimate would be about 26 characters per number (including commas and line-breaks), so: