Why does the XC60 require so much battery capacity
 

I have a 2015.5 XC60 with auto start/stop. The main battery is 800A, and I was just recently made aware of the auxiliary battery, with is 170A. So that's 970A total. I also have a 2021 Toyota Highlander, also with start/stop. It's a bigger SUV than the XC60 and with a bigger engine (3.5L v6). However, it only has a 605A battery. Why does the XC60 need so much more battery capacity than the Toyota Highlander?

You don't add the CCAs together since it doesn't use the smaller battery for cranking.

Maybe it's more than it needs... that's not a bad thing. Not sure why you're concerned with this.

Not a concern. Just wondering why. It should take a lot less power to crank my 3.5L Toyota than my 2.0L Volvo, yet the Volvo needs a bigger main battery plus an aux battery.

Also, in a sense you do add the CCA together. If I understand correctly normally everything runs off the main battery, but when restart start after a "stop" kicks in, the more sensitive components are switched to draw power form the aux battery. The remaining components and cranking draw from the main battery. The idea is to avoid a power drop to the sensitive components. I think without the aux battery you can see something analogous to your house lights dimming when the A/C first kicks in, and probably some of the car's components don't function well if this is allowed to happen.

As with all auto stop/start cars, it needs the extra power due to all the stop/stops .

Well that part of the point I'm making. My Toyota Highlander also has start/stop, does not need an aux battery, and the main battery is only 605A...and it's a bigger car with a bigger engine.