Perhaps you can wedge a physically bigger battery in there w/some custom fitting. If you try a different group size, make sure the +/- posts are oriented correctly & on the correct side, and that the highest point of the battery isn’t so high that it will contact the hood when driving over a bump. Consumer Reports does an annual battery rating. Which vendor/model tests out best depends on the battery group size, so that’s a good resource.
Do you actually have a problem getting the car to crank and start on cold mornings? Or it this more theoretical? If you actually have a problem, folks living in really cold climates, like Wyoming & Montana near where I used to live, I noticed they often use some type of engine compartment heater overnight that they plug in to an electrical socket to solve that problem. That might be a better solution for you than trying to wrestle in a physically bigger battery. Square pegs don’t fit round holes.
Note: Smaller engines like in a Fit often don’t require as much battery power as cars w/ bigger engines; and it also depends on compression ratios, starter/flywheel gear ratio, & whether the starter motor is a direct drive or a gear reduction version. So you can’t really compare the CCA needs of one car vs another directly. Just saying, so you don’t end up on a wild goose chase there.