Materials and textures.

By the way, I saw your signature and read about the changes.
The idea with the size seems very, very controversial, and frankly, it's not a good one. First, regarding the value of the battle arena, as the developers wrote in their diaries, the arena becomes more valuable when it's smaller, not larger.
Think about it, and you'll realize that this is quite logical. For example, with the large creatures. The fact that it might be inconvenient for you as a player to deploy them, and the fact that there's more area available for the opponent to attack them, all of this seems like something bad, but in reality, it forces you to find a way out of the situation. If this isn't the case, the game becomes easier and, most importantly, monotonous.
The moral of the story is that the game shouldn't be convenient in every way; there should be pre-created problems that you, as a player, will solve. If everything is simplified, you'll end up with 4 resources, convertible cities, and all the bonuses from all buildings being available for recruitment in any of the forts, basically like in Heroes 6.
Secondly, in my opinion, +1 to speed doesn't compensate for the reduction because (I'm not sure about this, but I think it's like this) when a large creature calculates its movement, it counts 4 squares as one step. If someone knows better, please correct me if I'm wrong.

In general, maybe I don't understand the idea; what's the point of reducing the size of the creatures?
If it's just to have more space, then it only sounds like a good thing at first glance.
Think about it yourself: if you hypothetically increase the size of the arena, what would change? Battles would become longer and more tedious; attacking units would lose at least one turn moving before they can attack, and ranged units would become more valuable and need to be rebalanced. Plus, considering that there's more space, it becomes harder to make a mistake, harder to execute some tactical maneuver using the proximity of units. That's all. I don't think it's "better"; at best, it's just "different," and most likely, it will even be "worse."

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Glad to help!