Everything is explained in this forum thread: http://forum.mctcp.com/t/is-this-meant-to-happen/494/2

In short, the issue is: when biomes with multiple sizes are in one climate zone, only the one with the lowest generationdepth-size-thing (the largest biome size) will generate. When I have a climate zone with EH (size 3, biggest) and Taiga (size 4), only EH will generate in that zone, even if I set its rarity (probability) to 1.

Something I'd like even more is this: being able to determine biome size per climate zone. I mean, the same biome having different sizes when generating in different climate zones (such as Plains having size 4 in one climate zone but size 5 in another). To make that work better, this issue should first be fixed. Should I create a feature request for the idea I posted here? @Timethor

@rutgerkok confirmed for latest version