ob_start buffering is dumping output prematurely

[JFlame]

I am trying to use output buffering in the following manner:

 

ob_start("OB_BUFFER"); 
echo 'Start';
do_stuff_function();
ob_end_clean();
echo 'End';

 

The problem is that when the output of "do_stuff_function" exceeds 4k the output is automatically flushed and I don't know why (exceeding some limit I suppose). 

 

So when the output of "do_stuff_function" is less than 4k, I get this:

 

Start
My <4k output here...
End

 

When "do_stuff_function" exceeds 4k, I end up with this:

 

My > 4k output here...
Start
End

 

I don't understand why this is.  The docs seem to indicate that the way I am starting the ob_start() function there should be no buffer limit.  Is that not true?