Re: "Tracer" MPPT controllers from China
"I'm not totally clear on why MPPT is only worthwhile with the larger wattage panels/arrays. "
Cost effectiveness, plain and simple. If an MPPT controller is going to buy you a 10-20% gain (pick your own number) compare the price of that controller to simply buying a 10-20% larger panel in the first place. If you can do that with buying ONE panel and simply making it a little larger, that's often more cost effective than spending the extra money on an MPPT controller.
OTOH if you're buying four panels and there's no more room on the boat but your energy budget demands more power, the MPPT becomes very cost effective, it may be the only way to get blood out of the stones. So to speak.