It's really interesting to see just how small that kingdom is; that would be the size of a county or department today. I wonder if all of these areas had kingdoms that small, each bordering one another, slowly coalescing into ever-larger kingdoms and polities until today.
It's been argued that countries/empires are naturally limited to about 1 month travel time from the capitol city.[0] In this view, the Roman Empire could grow so big mostly because the Roman roads were so superior!
This would explain the small size of Bronze Age countries. As transport technology gradually improves, we'd expect the size of countries to enlarge.
That makes sense, except in the case of colonial empires. Maybe since those empires were fairly diffuse and tended to be ruled from local sources of power (e.g. Calcutta for British India), they were still only 1 month away?
And, it seems that this kingdom was much smaller than what one could even walk in 1 month, even if there were no roads. 30km x 25km could easily be walked in a day by a relatively fit person at a reasonable pace. So maybe it isn't just travel time, but things like whether the state has enough power to prevent banditry (correlated with distance from capital) or distance-as-measured-by-armies-and-tax-collectors.
I believe the argument was that countries couldn't be larger than 1 month travel time. It's a maximum size limit, not a minimum.
Also as others have pointed out, I think you'd be surprised how difficult and slow it is to carry heavy army-supplying loads over a winding network of unimproved trails that periodically get flooded out or turn to mud, all while being assailed by bandits and toll collectors. Travel in that era was not only slow and expensive, but risky.
I think you’ll find 25-30km a day is very optimistic once you need to account for people having to make and break camp and forage for supplies along the way too and you need daylight for that.
Those distances might be doable within the developed areas of an empire when you are moving between established settlements or way points with established infrastructure but not when there is no where to rest and no where to eat.
Have you hiked off-road with backpack? Many are exhausted after 15km, and that's with good shoes and good backpacks.
To make it worse, a distance you can do in 1 day isn't the distance you can do every day for weeks, which has to be less. And it's always possible someone pulls an ankle or the rain comes, which will further lengthen the trip.
Of course, but Roman roads were far superior. They were more durable (especially under heavy traffic for commerce or supplying an army), straighter, had good drainage, and they linked together to form a complete network.[0][1]
Earlier paved roads do exist,[2] but (due to their high material/labor cost) this was only sporadically feasible, between settlements with high traffic between them. Part of the genius of the Roman road network was the system of bureaucracy and taxation to pay for it. This meant that a traveller (or an army) could rely on the existence of a continuous, high-quality road linking distant locations.
