When is the "little" userbase excuse going to go away?
PSP 2006-08-07 3,825,233
SD Gundam G Generation Portable (PSP) - 155,634 (2006-08-03)
3DS 4,084,190
SD Gundam G Generation 3D (3DS) - 90,873 (2011-12-22)
I think there are other factors involved besides platform userbase as well. While some titles released early in a system's life could get a boost for simply being available, there are also titles which can be overlooked and ignored early in a system's life if the demographic for that particular game isn't fully on board the system yet.
Let's compare the two DS G-Generation games for example.
SD Gundam G Generation DS (DS) - 53,000 (2005-05-26) <-- Released 6 months after the DS launched, the DS userbase was 2,435,033 at the time.
SD Gundam: G Generation Cross Drive (DS) - 137,717 (2007-08-09) <-- Released almost 3 years after the DS launched, the DS userbase was 18,846,562 at the time.
I don't think the specific number of people who own the platform automatically dictate the userbase a game has. Increasing the DS userbase by another 16 million people certainly did not result in the next DS game selling 8 times more. But at the same time, the increased userbase did mean that it was likely that a huge percentage of the people who did buy G-Generation games who did not own a DS when the first game was released, had bought a DS by the time the next one was released. This is where userbase is meaningful.
The 3DS might have more overall owners than the PSP did at the time G Generation Portable was released, but we have to consider that the PSP was 2 years old by the time that game was released. This made it much more likely that over time, people who liked Gundam games have the chance to pick up the PSP, and there is a larger base for those gamers in the overall userbase. For the 3DS right now, that is obviously not the case.
If the userbase of the 3DS increases dramatically over the next year and more Gundam games are released on it, a subsequent exclusive G-Generation title on the 3DS would almost certainly do better.