Adding data to a text file with SED not changing the file size - sed

Question

I have some text files where I need to add 1 character to the beginning of every line of the file.
In windows, I found that a quick way to do this was by installing Cygwin and using the following command, which prepends the letter N to every line of the file:
$ sed 's/^/N/' inputFile.txt > outputFile.txt
What I found strange, was that after I added a new character to the front of each line, the file size was almost completely unchanged. I tested this further, to see if I could recreate the problem with the following steps:
Created a text file called "Test.txt", which had 10,000 lines with the word "TEST" on each line.
Created a text file called "TestWithNPrefix.txt" which had 10,000 lines with the word "NTEST" on each line.
Executed the following command to create another file which had 10,000 lines of "NTEST"
$ sed 's/^/N/' Test.txt > "SEDTest.txt"
Results
"Test" and "SEDTest" were almost the exact same size, while "TestWithNPrefix" was 10KB larger.
Test = 59,998 Bytes; SEDTest = 59,999 Bytes; TestWithNPrefix = 69,998 Bytes
When I ran the "fc" command in Command Prompt, it returned that there were no differences between "SEDTest" and "TestWithNPrefix". "FC" between "SEDTest" and "Test" returned "Resync Filed. Files are too different".
Can someone please help me understand what is causing these file size discrepancies?
EDIT: I created the files "Test.txt" and "TestWithNPrefix.txt" in UltraEdit. I just typed out the word "TEST"/"NTEST", then copied and pasted it 10,000 times.

Answer 1

Not an answer, but a comment with formatting:
You seem to be running into some odd situation with DOS versus Unix line endings. I have to ask: How are you creating the files? I would expect 10,000 lines of "TEST\r\n" would be exactly 60,000 bytes in size, not 59,999
On Linux (I don't have access to a cygwin environment at the moment):'
$ yes $'TEST\r' | head -n 10000 > Test
$ ll Test
-rw-r--r-- 1 jackman jackman 60000 Jan 8 13:06 Test
$ sed 's/^/N/' Test > SEDTest
$ ll *Test
-rw-r--r-- 1 jackman jackman 70000 Jan 8 13:06 SEDTest
-rw-r--r-- 1 jackman jackman 60000 Jan 8 13:06 Test