WordPad limitations

Does anyone know if WordPad can only display a certain amount of info?
I've got a huge dat file that I need to seperate into smaller dat files in order for this preprocessor to be able to parse them without crashing, so I opened up this 139 mb dat file in word pad, copied and pasted sections of it into smaller files. Then wrote a quick program to count the lines in each to make sure they match the original. I'm getting totally different results, like the visible data I copied and pasted came out to 66 thousand lines, where as the original file comes out to over a million lines. Now, in looking at the contents of the variable where I input the data into, at line 1 million, it shows data that I cannot see in the word pad file.
Long story, sorry. So, I'm wondering if WordPad is only opening so much of the file without warning me that it cannot open the rest.
I have not tried programatically to break up the file, I guess that's next.
Just very curious if anyone knows of WordPad limitations in this area.
Whew, if you made it this far, thanks for reading. ;)

Thanks in advance.