I am creating a fantasy world with high presence of magic. I wanted to use large versions normal animals as mounts used like horses in war and normal life. My explanation as to why there are larger versions normal animals is that sorcerers have used magic to alter their anatomy to become larger and began mass breading of them. It can also be done to birds so you can have an air force made of giant eagles, falcon, hawks, and more. Does this sound like a good idea? Is my explanation of it plausible? Any tips for improving it?
Magic can make anything plausible. The key is to think about consequences. If magic is commonly used to enlarge animals, it won't only be used for war. Will gaint oxen pull ploughs? Giant sheep grow wool? Giant rats infest barns, and require giant cats to keep them under control?
In thinking about how a technology (and wide-spread magic use is a technology) affects the economy, the two most important things to consider are "who stands to gain because this technology exists?" and "who stands to suffer because this technology exists?"