Easy reproducible; create an edge biome in random mode and play around with BiomeSizeWhenBorder, then change BiomeSize (which shouldn't do anything to a border biome) and watch it change.

Isle biomes also use BiomeSize / BiomeRarity and not BiomeSizeWhenIsle / BiomeRarityWhenIsle.

Using TC build 163 in Spigot 1.10.2 in BeforeGroups mode.

I should add a note that those settings don't work in the BeforeGroups mode.