Ummm wasn't dragon quest 8 on PS2. Doesn't PS2 have a bigger install base than the DS. hell isn't the PS2 the highest selling single console to date.
Heh, wouldn't it be great if everything was that simple? There are a lot of other factors, such as attach rate, popularity, competition, and of course the other games in the library. But, hey, just for the hell of it, let's go with your theory. Graphs are fun though, so lets play with this one.
Here we have a graph comparing current Japanese sales of the PS2, DS, and PS3. Time for some fun math! After only 2 and a half years, the DS is a mere 3.8 million units behind the PS2's 21.9 million sales after 7 and 1/4 years. This places the DS 17% behind the PS2's sales. The PS3 on the other hand is currently sitting at 1 million units sold is currently 95% behind the PS2's sales, but since it won't be released for a while, I'll be nice and go ahead and DOUBLE the PS2's current Japanese sales (something which is very unlikely to happen by the release of FFXIII), putting it at 90% the PS2's sales when FFXIII comes out.
Since you are so keen on amount of console sold, let's apply these differentials to the 2 games which are primarily being discussed (Final Fantasy XIII and DQIX).
Dragon Quest VIII sold 3.61 million units in Japan, if we apply the -17% to it (as per your console sales theory) then DQIX will sell about 3 million units.
Final Fantasy XII sold 2.42 million units in Japan, if we apply the -90% to it then FFXIII will sell about 242,000 copies.
Now, we both know this isn't the way things work, and although FFXIII will very likely sell around FFXII's numbers or less, I very much expect DQIX to be the best selling DQ game in history (both in Japan and Worldwide).