The battery specs describe two different things about the battery.
CCA(Cold Cranking Amps) describes how much current and thus, power, the battery can put out at one time. This is a function of two design factors, the surface area of the plates, and the resistance of the structures that connect the plates electrically.
Amp Hours is a function of how much lead is used in the battery. More lead, heavier battery and more amp hours.
Low amp hours and high CCA is possible. It just means that you won't get them for as long as a battery that has more amp hours. The difference is going to be proportional to the difference in amp hour capacity. That is, a 20amp hour battery with 200CCA will crank the engine for about 80% as long as a 25amphour battery with 200CCA.
The typical battery putting out 250 CCA starting an engine that starts in 1 second of cranking will consume 1/3600 of the battery capacity to start, or about 0.07amp hours(that's hardly any). You should be able to get about 20+ seconds of cranking from a 20amp hour battery before the voltage drops to where the CCA can't be delivered. A 30 amp hour battery should be able to provide an additional 10 seconds of cranking before it won't continue.
Bottom line is that as long as your engine starts easily the amp hours of the battery is not that important. However, if you like to stand around with the lights on before you decide to start your engine The bigger battery will be better.
One advantage of the bigger battery is that as batteries age, they will lose some of their capacity. A smaller capacity battery will become unusable sooner than a larger one. Good maintenance practices will prevent loss and a well maintained battery will outlive a neglected battery of higher capacity. The most important factor for maintenance is to keep it fully charged and plates submerged in electrolyte. Every premature battery death I have seen in my line of work has been directly caused by failure to keep the battery charged when not in use for long periods. GET A BATTERY TENDER AND USE IT!!! Even the cheap ones will extend the life of your battery if used regularly. Just don't leave a cheap one connected for more than 1 or 2 days.