Size in byte of file reduced when i combine two csv

Here is code that can first check the length of file if it is less then 800 it will do following steps first it will get the data from 1-600 rows in a Variable then using same csv it will get 1-200 rows in a Variable save both variable in new files and at the end combine them but issue is that my file size in byte is reduced when ever i check the data in cell i observe that values are truncated. can any one have another logic for this work

home_dir = 'D:\DS\Testing';
sensors = dir(home_dir);
for sens =3:length(sensors)
    classes = dir(fullfile(home_dir, sensors(sens).name));
    for cls=3:length(classes)
        data = dir(fullfile(home_dir, sensors(sens).name, classes(cls).name));
        toWrite = cell(zeros(1,1));
        for dat = 3:length(data)
            
            tmp = readtable(fullfile(home_dir, sensors(sens).name, classes(cls).name, data(dat).name));
            
     
             n_rows_target = 800;
             n_rows = size(tmp,1);  
            if n_rows < n_rows_target % Check the file length
                
                
                B=tmp(1:600,1:end);  %Get the data from 1-600 rows 1-4 Columns
                C=tmp(1:200,1:end);  % Get the data from 1-200 rows 1-4 Columns same file 
                
                
                writetable(B, 'new.csv');  % File is created in which 1-600 rows exist
                writetable(C,'Two.csv');   % File is created in which 1-200 rows chunk 
                
                csv1 = readmatrix('new.csv'); % Reading Files
                csv2 = readmatrix('Two.csv');
                allCsv = [csv1;csv2]; % Concatenate vertically means 1-600 
                                      %                              1-200
                                      %                              800
                                      %                              Rows
                
                writematrix(allCsv, strcat(strrep(data(dat).name, ' ', '_'), '')); % New File is created with 800 rows
            end
        end
    end
end   

[Stephen23]

As an aside, note that these lines of code:

sensors = dir(home_dir);
for sens =3:length(sensors)

are a buggy attempt to deal with the dot directory names. Robust code uses SETDIFF or ISMEMBER.

https://docs.microsoft.com/en-us/openspecs/windows_protocols/ms-fscc/fccd0313-0364-45bd-b75c-924fd6a5662f

https://www.mathworks.com/matlabcentral/answers/484430-using-struct-dir-selpath-what-do-and-mean-as-struct-name#answer_395754

https://www.mathworks.com/matlabcentral/answers/40949-omitting-pointers-when-listing-folder-contents#answer_50568

https://www.mathworks.com/matlabcentral/answers/13978-list-of-files-sorting#answer_19278

As Walter Roberson wrote in that last link: "In short: if your code assumes that '.' and '..' are the first two entries in a directory, your code has a bug (even in MS Windows). If your code assumes that directory entries are returned in any sorted order, your code has a bug (in all OS.)"

"when ever i check the data in cell i observe that values are truncated"

Nothing in your code seems to "truncate" values. Please upload two sample files by clicking the paperclip button.