I don't get it... system costs scale pretty closely with number of panels.

If a $X system covers 100% of your use, a $X/2 system probably covers 50% of your use.

Ignoring time value of money, if the $X system saves you $X / 5 per year, the $X / 2 system that generates 50% will save you $X / 10 per year. Both systems would have a 5 year ROI.

Even if your half price system generates only 45%, it only brings the ROI out to 5 years 6 months.

There's are some fixed costs to hook into your panel and whatnot, but as long as your system is within a reasonable range, the math should math pretty well until your system generates more than your usage. Assuming the math maths at all... if system costs are high and utility rates are low, it doesn't math. If you roll a roof replacement into your system cost, then yeah, a small system won't get you the ROI, but I don't think the math works to include roofing in the system cost unless your utility power is very expensive per kWh. If you're in a reasonable cost area, and you need a new roof for your system, you need to wait until it's time for a new roof anyway. Or if you can't afford a new roof and a solar system at the same time and don't want to finance the solar system, you could probably have the roofers put in rails for future panels when the roof is installed so that the roof warranty covers the rails. Then you can add solar when you've rebuilt your reserves.