Now that we've cleared up that it is possible to fit everything on a Wii disk the only reason why the Wii cannot play the same games that the 360/PS3 can is purely down to processing power. If the Wii can't run a game engine then the game has to be cut down to allow it to run.
COD:WAW is a good example, it used the same engine as the 360/PS3 versions but the cost of running the engine was that some of the content had to be left out (either that or lazy development).
RE:5's engine doesn't look much improved over RE:4 to me, the only real change being online/co-op modes. They could just reuse the RE:4 engine and cut some content to make a quick and easy Wii specific RE:5.
Now people may be against an altered Wii version but it could actually make for a better game. RE:5 lost any 'survival horror' aspect and ended up being just another 3rd person action shooter. RE:4 was more 'survival action' with a bit of horror, so changing RE:5 to something more like RE:4 could be a good thing.
The Wii already has the best RE:4 version, archives coming out and the Wii exclusive rail shooters (as well as playing the NGC games) so if they did reformat RE:5 over to the Wii then it could end up as being the best system for the entire franchise.