Deka has a large line of batteries, so I'n not sure exactly which one you are referring to:http://www.dekabatteries.com/assets/base/0083.pdf
A miata battery is minimal at 320cca and 25 lbs.
A replacement battery for a Volvo 240 is 600cca extra 7 lbs.
Deka suggests that using a battery with less than the minimum cca will shorten the life of the battery.
More cca is always better as long as the cables are adequately sized.
The charging system keeps your accessories going.
The battery is for starting and to keep you moving when the charging system fails (reserve capacity).
Fewer cca also means less rc so while a 240 may only need 450cca, the amount of accessories in a volvo push the rc requirement up and the cca with it. A 5.0L mustang needs only 580cca.
The Odyssey 680 is around 220 cca apples to apples (i.e. 30 seconds at zero F): http://www.odysseybatteries.com/odyssey101.htm
A PC925 is appropriate for a miata 1.8l.
A PC1200 is appropriate for a stripped down volvo 240 2.3l.
Check out the specs at the bottom of the page:http://www.odysseybatteries.com/batteries.htm