I found a way in Windows to do such thing

echo "This is just a sample line appended  to create a big file. " > dummy.txt
for /L %i in (1,1,21) do type dummy.txt >> dummy.txt

http://www.windows-commandline.com/how-to-create-large-dummy-file/

Is there a way in UNIX to copy a file, append and then repeat the process? Something like for .. cat file1.txt > file1.txt?