Materials and textures.

Secondly, in my opinion, +1 to speed doesn't compensate for the reduction because (I'm not entirely sure, but I think it works like this) when a large creature calculates its movement, it counts 4 squares as one step. If I'm wrong, someone who knows better can correct me.

So, what's the point?
When something moves one square, it moves one square.
Another issue is that large size is actually a disadvantage for a creature in most cases (sometimes you can use a large creature to block the enemy's path or prevent them from effectively using dragon's breath, but these are exceptions). Therefore, removing the "large creature" property and giving +1 speed in return seems questionable. Especially for liches – imagine how much easier it is now to deploy them in the arena without exposing them to fast-moving enemy creatures.
But overall, these are interesting changes.