Yes, they are, as a matter of fact, big numbers. For whatever reason, the battles of antiquity significantly outsized most battles of medieval times. Don´t ask me how.
It's a complicated story with a lot of factors, but from my understanding, a big factor is Rome being Rome, and Rome had a very special approach to both its social organization and its military organization.
Part of this might be easier to explain by pointing at what Feudal Europe lacked: a large professional administrator bureaucrat class. Feudalism developed as a means of doing without administrators, by handing off large chunks of land to vassals, who in turn hand off smaller chunks to their vassals, until you get the King-Duke-Count-Baron hierarchy system known from Crusader Kings (obviously greatly simplified) as a means of reducing how many people each guy in the system has to interact with, and how much each of those guys has to keep track of.
(Priests and merchants offered plenty of well-educated administrators, but warrior nobles were very skeptical of bringing large amounts of these other classes into power.)
Feudalism gets replaced by absolute monarchy around the time the kings can get in a bureaucrat class to do the management for them, replacing the middle vassals. Then the bureaucrat class replaces the kings in turn, producing the modern absolute republic and absolute democracy.
The Roman Republic was in a sense an early absolute republic, before becoming an absolute monarchy. "Absolute" here does not mean very republican, it means with
absolute power, perhaps a better word might be "totalitarian", overseeing its subjects in great detail and extracting a lot from them in tandem with doing a lot for them. You know the Monty Python joke: "All right, but apart from the sanitation, the medicine, education, wine, public order, irrigation, roads, a fresh water system, and public health, what have the Romans ever done for us?" - and the Judean People's Front is still rebelling because the Romans are still also oppressive. The Romans are engaged in what might be called intense cultivation of their population: requires more work, generates more output, compared to grazing and gathering. When this system breaks, it is not easily fixed.
Then there's the Roman Legions. Fortification-specialized heavy infantry get to dictate the pace of combat. On a tactical scale, heavy infantry beat almost every other unit commonly seen in play. (Heavy cav is numerically rare, horse archers are geographically rare.)
Light infantry: just crush them
Light cavalry: close ranks, crush them
Foot archers or slingers: shields up, crush them
On the strategic scale, states with these other units will try to pick different ploys rather than facing a legion head-on, but most of
those ploys are in turn countered by the legion going "overnight fortification LOL". You want to hit them in the logistics? To get at their logistics, you have go among Roman forts, and the light infantry will have second thoughts about going anywhere that their line of retreat has an enemy fort sitting on it.
Sure, legions have drawbacks. They're slow, they're expensive, they require industry and long training, but the big advantage is that
Rome got most of the choice in which fights it wanted to have, and it could recruit auxiliaries and allies to fill all the roles other than heavy infantry.
If some state with Chariots wants to have a fight, and Rome doesn't want that fight, the Legions simply sit in a fort every night and point and laugh at the inability of chariots to get over even moderately sized walls and trenches. The Legions are prepared to sit in a fort. The Chariots are
not prepared for a siege.
Combined, these two things create a selection bias for small fights not happening. When the Roman Legions roll up five thousand strong into the small Example Country, and the King of Example finds them three days into fortifying on his doorstep by the time he can raise an army and muster a response, he's going to have a hard choice between 1) attacking fortified heavy infantry, plz no, 2) letting a legion of heavy infantry run around in "his" territory, plz no, 3) trying to besiege fortified heavy infantry when he brought no siege engines and the Romans can bring another legion to break the siege, plz no, or 4) become client state of Rome and he gets to keep his title.
A lot of petty kings picked 4.
They even get to pretend they're independent! They're not vassals, vassals have to bow and scrape and be utterly subjugated, King Example is a beloved
ally of Rome who is merely
strongly expected to send troops when Rome calls for aid because that's what allies do. Of course if he ever tries calling for aid the Romans will be very sorry they can't send anyone because they're busy fighting like 15 other wars at once.
So where medieval battles "had to" happen in a sense, for uncertainty and lack of control and unfavorable terms for everyone, the Romans got to skip a lot of that and focus on the larger battles. Everyone else bordering Rome, then, had to either muster similarly large legions any way they could, or else get stomped so hard it barely gets counted as a battle.