The other thing to remember is that you really shouldn't try and align fantasy mass combat to actual medieval mass combat. They wouldn't evolve the same way, due precisely to the spellcasters and the "fireball" effect.
If you really wanted to run fantasy mass combat, it would actually turn out and look much more like the combats of World War II than any of the medieval "two sides stand 500 yards apart out in the open and then charge at each other" kind of battle like in Braveheart or whatnot. Fantasy mass combat would be the kind of fighting that had constant bombs going off, people diving into trenches and foxholes to avoid explosions, and much more hit and run or small tactical band combat like you'd find in Saving Private Ryan or Band of Brothers. Spellcasters are basically snipers, grenadiers, machine gunners, and bombadiers.
Bombs and incendiaries weren't invented in the 20th century. Such weapons have existed since ancient times, and cannons and knights coexisted for quite a while. What makes the difference in modern warfare is
mass production. Billions of artillery shells were fired in the World Wars, each one of them the equivalent of a 3rd-level spell slot*. Unless your army can field a corps of 18th-level evokers, wizards don't pack anywhere near enough ammo to recreate WWII. Furthermore, while artillery can hit targets miles away, the range of
fireball is a mere 100 feet. Longbows outrange it by a factor of six. You have to be practically on the front lines to cast it, exposed to every archer the enemy's got... and if you order friendly soldiers to clump around you with shields, you invite a
fireball from your opposite number.
A low-magic setting with few wizards would look a lot like late medieval/early Renaissance warfare, with wizards more or less substituting for cannon. A setting like Forgotten Realms would look like neither medieval warfare nor modern; it would be completely its own beast. Even in FR, there aren't enough wizards to recreate a modern battlefield, but there are more than enough to transform warfare by acting as scouts, saboteurs, infiltrators, and assassins. It would be like war with medieval weapons but modern information technology.
*Edit: After some more thought, I realized this is the wrong comparison. An artillery shell isn't a
fireball--it's a
meteor swarm. The
fireball is essentially a grenade. So your 18th-level evoker can play a howitzer for 2 shots per day; then she's just a grenadier.