I observerd it one time. It just overrides any Previous Biomes. Dunny Why. Just happens, maybe a Bug?
On a Technical standpoint I can understand why it does that.
You have to Imagine your World like a Painting. TC paints the Biomes/Groups.
It starts with Depth 0, Where it (in most configs) first Draws the LandMass and Divides it into Ocean/Land.
Then it Draws the BiomeGroups at Size 0. Then the Biomes inside those Groups, at Size 0, then Isle Biomes inside the Biomes where they are supposed to Spawn, and then the Border Biomes, basically everything at size 0. (The Order may be different, maybe there come first the borders then the Islands or something, I dont know the Technical Standpoint)
Then It moves on to Size 1, and does everything the same.
Now, when we go to BiomeGroups, if we fill our BiomeGroup with a set of Biomes, We have a area of Biomes, now we move to the next Size, and we have only one Biome at this size for that group. Then It will just overwrite the whole Group. Maybe its not intended, and it is only Supposed to Draw a bit over. Maybe it can be tuned with the Rarity, and it works with a Low Rarity. That would be something one need to test out.
Not neccessary. if you make Plains size 4, Forest size 5, and extreme Hills Size 6.
Which in return should make the Extreme Hills smaller than the Plains by a fair Bit. It may still happen occasionally that the ExtremeHills will be Larger than the Plains. Because the "Biome Size" it not exactly the Size, rather than the Layer/Order/BrushSize
You can still Draw with a tiny Brush a Large Circle.
To Get more Control over the Actual Sizes of Biomes you should work with Isle Biomes and Border Biomes. These give faster and easier the desired Results.
For Example, the Default Configfiles.
You get in your BiomeGroup for example: Plains, and Forest.
The Forest is also a Isle in the Plains Biome, that way you dont get just two Biomes, next to each other, rather you get plains with the Occasional small forest, with Larger Forests.
You can get that a Step further, you can have BorderBiomes, on IsleBiomes, which have Border Biomes where other Biomes are an Isle inside.
You cant get that varierity in a BiomeGroup.