how to make binary file smaller

Hi

I'm writing parameters gathered through a function to a txt file when certain occurances happen. I prepend one parameter to uniquly identify the line being writen and then I close it (I have to close it). My problem is it seems like every time I open and close it I get a bunch of white space in the text instead of just appending to where it left off. This appears to inflate the file to 5 times the size of what it should be.

I'm attaching the output but the source code is on a different machine right now.

I don't want to use 3rd party compression at all I just want it small from the begginning. Any Help?

Thanks
JoeyO