A step-up from TiddlyWiki that is still 100% portable?

Indeed TiddlyWiki can work with VERY large tiddler stores, they don't need to be in the current TiddlyWiki document either.

See "import tiddler" and friends over at http://tiddlytools.com

After some time and serious consideration, I will post my own answer.
There is nothing that matches TiddlyWiki.

As for voluminous information, TW can pretty much handle it. (My early discouragements were due to malformed code.) Difficulty accessing information through the interface becomes an issue before any speed problems. This isn't to fault the interface -- it could be more powerful, but that would sacrifice lightness.