The problem with mass combat is that the individual contributions of one character really shrink- if 10,000 men are fighting another 10,000, surely a 20th-level Fighter should be performing epic feats on the level of Samson taking out 1000 Philistines all by his lonesome, but the rules of the game don't really reflect this- maybe if all these 0-level guys are less than 1 Hit Die, an AD&D Fighter could take out 20 in a turn, letting him do that over the course of 50 combat rounds, but that's probably the pinnacle of what that character could do by their lonesome. They don't provide leadership bonuses on that scale innately (you'd have to add those, but you know, that would beg the question of why they don't inspire their party the same way). Sure, back in the day, a Barbarian could raise a horde, and Fighters, while their followers fell far short of an army, still had some people they could bring with them. But in a mass battle, what characters would be doing is special missions, like a fantasy A-Team, gathering intelligence, taking out enemy leaders, disrupting the chain of command and so on- which while very important and more likely to turn the tides of battle than wading into a mass melee, is probably not the kind of visceral experience a player would want- they want something more like Toei's Warriors series, where one guy runs around the battlefield, bowling over hundreds of foes.
And I hate to bring this up, because I risk dredging up a lot of old arguments, but there is an exception- high level spellcasters. They often have big area spells and the ability to impede or kill large swathes of the enemy.
In fact, thinking about it, this might have been what Gary Gygax was worried about when it came to spellcasters more than their contributions in the adventure- when a high level Druid can eliminate 1000 hit points of enemy troops per casting of creeping doom or pinpoint enemy leaders with call lightning, or wizards dropping 4 40-foot radius spreads of damage with meteor swarm, etc. etc., you really start to realize how terrifying they are on the battlefield, dealing damage equal to a siege weapon, but much faster and with greater accuracy!
I remember a 3.5 game where the DM had us defend a village from goblin raiders- he assumed we'd winnow down their numbers a little, but our victory condition was merely to slow down their advance so reinforcements could survive. He had this huge battlemap set up at the local game shop. I was playing a Druid.
I got out a ruler and some string and started making these huge areas on the map and he stopped me and was like "what are those?". "Well, the green ones are spike growth, the brown ones are transmute rock to mud, and the white ones are stone spikes, all based on the terrain. I got these yellow ones for soften earth and stone, and I have a few holly berry bombs at these choke points using fire seeds as good measure."
He laughed at me and said I'd gotten the scale of the map wrong and went and checked my spells. He got this horrified look on his face when he realized exactly how big some of these spells can be- 12 20' radius squares per casting of a spell really added up! And that was just what I was doing, by the time he got to the Cleric and the Wizard, he realized exactly how futile things were.
We really got into an argument when I pointed out that spike stones and spike growth were considered to be traps and had to be searched for. "If someone sees their buddy walk into an area and take damage, they know not to go there, even if they can't see what's hurting them!", using this to justify having his goblins avoid them- but I'd figured something like this was going to happen, so the goblins were just funneled to a worse tactical position.
In the end he said they would just go off the map around us- even though he'd already shot down (pun intended) our Ranger who pointed out he could fire arrows from off the map at distant targets (and make the Perception check to see at that range, thanks to his eyes of the eagle, all so his reinforcements would be the ones to save the day, lol.
Because the way we were set up, no goblin was going to get through us.