Hope this helps to explain the differences in metal size and why it is needed.
The smaller topiary frames use a smaller gauge wire rod (1/8″ or less) as they do not need much support for the structural weight. The larger scaled topiary frames, such as a horse, require a larger gauge wire rod (1/4″ or larger) as they are heavier and need more structural support for weight and stability.
Picture of smaller topiary using 1/8 inch gauge wire.
Any open wire topiary frame that does not have a covering (foliage) should withstand normal wind conditions. Large-scaled topiary frames that are covered or will be in high wind environments would need additional internal supports that connect to its base. This is typically done with a larger rod. In this specific case of a horse in 70mph winds, the 1/4″ wire rod with one or two 3/8″ rods for internal structural support would work just fine. The base could be bolted down to a concrete pad/footing. If the horse will be covered, then it may need a few more 3/8″ rods for additional support/stability. Here is a picture of a topiary using a 3/8 inch gauge wire.
Our is my recommendation for longterm outdoor use with the worst possible wind conditions. Anything smaller than the 1/4″ wire rod and no internal support runs the risk of being damaged.