Battery sizing can be a little confusing because the numbering system isn't entirely sequential.
Group 34. Common to compact racing batteries and AGMs like an Optima. Similar to a group 24 but not as tall.
Group 24. A standard size for starting and marine batteries. Stock in most towboats.
Group 27. Larger dimensions and capacity over a group 24. Requires a larger 27 case/lid or tray/bracket.
Group 29. For all purposes this is the equivalent of a group 31.
Group 31. Larger dimensions and capacity over a group 27 and will often fit in a group 27 case or bracket.
Its really simple but usually as the battery becomes heavier and larger the battery has more reserve capacity. You could almost skip the amp/hour rating and go purely by the weight.

David
Earmark Marine