Weight and appearance
Size 26F is the smallest and cheapest lead acid readily available with the most CCAs, enough for the starter. There are smaller and lighter batteries out there but not worth the extra cost I've found to save a few pounds
outboard of the rear axle, that defeats the concept of a mid-engine in my opinion. It's a big (ugly) heavy weight out there, not ideal. I'm not setting up for racing, but I like to think about those elements. Firewall is central and passenger side attempts to balance weight when driving solo. These are small things but when you have the freedom to choose, can't help but think about it.
did I mention the battery is ugly. I'd rather show off that pretty MagnaFlow logo and welds on those mufflers.