Install base does not matter at all. If you're going to ship for multiple platforms, you're going to have to deliver on all platforms. The relative number of units sold does not really matter for the development process. You could question if people should make games for the PS3 at all, as it has a smaller install base, but that would be missing the point. Old economics trick question: If you have two investment options, A and B, and A returns more money than B, which option do you take?
Even though you brushed it off (and rightfully so), I just don't see how a company would even question as to whether or not to develop a PS3 version as the PS3 sales would far outweigh the production costs. I simply can't fathom the ability for a company to leave that much money on the table. And when it does happen it's usually related to compensation received as a result of exclusivity or timed release deals. To tell your stakeholders that you're going to ignore a market with a 25% less installed base would not sit well unless you have a perfectly legitimate reason.