Notepad++ doesn't support huge file, according to this wiki documentation on SourceForge, the problem persists if the component (Scintilla) remains as the core of notepad++:
Older versions of Notepad++ may crash
while opening very large files. Newer
versions (5.2+) will deny opening the
file and issue an error message. Why
Notepad++ is based on a component
(Scintilla) which is geared towards
providing rich ext viewing, with
syntax highlighting and code folding,
as opposed to bulk text services.
There are necessary trade offs.
Loading a 200Mb file will require
around 800Mb of memory, and the OS may
balk at the memory allocation request.
There are basically two ways out:
Split the huge file into manageable chunks, and keep only one
of them in editor;
Use another tool which is tailored to handle massive text files.
If you still manage to open a large
file, performance may not be optimal.
Here is a couple things to think
Plugins which analyse and scan text will slow down NP++, disable them
Parsing for clickable links on loading the document is slow if the
document is large; disabling the
clickable links has been reported to
Another page on sourceforge also suggest this problem persist through out notepad++'s life, as the community request to solve this problem:
Why don't you try other software such as gVim? Is there any reason?