However, another possibility is the the regexp is consuming lots of memory.

The regexp seems simple enough (b'.'), so I doubt it is leaking memory like
mad; I'm guessing you're just seeing the OS page in as much of the file as it
can.

Yup. Windows will aggressively fill up your RAM in cases like this
because after all why not?  There's no use to having memory just
sitting around unused.  For read-only, non-anonymous mappings it's not
much problem for the OS to drop pages that haven't been recently
accessed and use them for something else.  So I wouldn't be too
worried about the process chewing up RAM.

I feel like this is veering more into python-list territory for
further discussion though.