It's 40MB, not 20, for starters.
I understand (though could be wrong) that it's the download package size, but also the size the game takes up on the Wii internal storage. You can't 'unzip' the 40 MB to take up 200MB like a PC installation. Any decompresing is done at run-time.
Some games take heaps of blocks for savegame data (especially if you're recording audio/video on a karaoke game), but they may be disc titles only, not Wiiware I'm thinking of. There may be a savegame data limit on Wiiware titles.
At least one Wiiware game has add-ons - Final Fantasy: My Life As A King.
http://wiiware.nintendolife.com/revi...life_as_a_king
Quite how that works, I don't know, nor do I know if that situation can be utilised by Telltale to, for instance, add a 'speech pack' as a separate entity to the base game which could be subtitles-only.
But considering what Telltale have done with getting everything packed into 40MB, having one lot of 40MB for the graphics, music and sound effects, and a 2nd 40 MB for the speech would likely make for a very nice experience for all.
The other consideration is that even though Wiiware titles can now be stored on the (up to 32GB) SD card, when they are played they actually copy over to the system memory. That memory is also used for some Nintendo channels, Disc based savegame data, the operating system (?) and a few other bits and pieces. So even though there's less issue with buying lots of Wiiware titles and saving them on the SD card, there is still the issue of the small internal storage (512MB) and that any Wiiware game has to take into consideration all the other things on the internal memory... which is why it's not so simple for Nintendo to up the Wiiware size limit from 40MB to 100MB.