Size
The strength of 1/2-inch diameter galvanized steel pipe depends on how thick it is. Inside diameter may be expected to be 1/2 inch, but this is not the case, since most inside diameters of 1/2-inch pipes are usually more than 0.6 inches wide. This may sound confusing, but over decades pipe specifications have changed with technology and materials, while the old labels like "1/2 inch" are still used. So despite the variations, pipe with a small diameter, like 3/8 of an inch, will still generally be stronger than 1/2-inch pipe.
Variations
Not all galvanized piping is created equal. While the inner diameter of the pipe may be set and the materials limited to only a few options, thicknesses can vary slightly, and quality of materials can affect how strong the pipe is. The type of pipe component also plays a part -- some pipe pieces, like joints, may be constructed to be stronger than other pieces. The strength of a pipe might also vary from place to place along the length of the pipe.
WOG
WOG stands for water, oil and gas. When galvanized pipes are constructed, they are rated for WOG limits, or what pressures they can carry these three materials at. Water, oil and gas are all similar enough that the same rating can often be used for all of them. Galvanized pipe is often rated at WOG 150 PSI, where PSI is the typical pressure measurement for pounds per square inch. Higher than this and the pressures will be able to damage the piping much more easily.
Steam
Steam has a separate rating than water, oil and gas. Steam under pressure acts differently than water or gas, combining some of the qualities of each under high temperatures. For 1/2 galvanized pipes, the difference is generally minimal and steam is also rated at 150 PSI, which can save time when fitting steam boiler systems.