It's always a balancing act. Using all big cores would eat up die space. With three big cores, developers should be able to place their primary threads on these cores and use the others for minor tasks.
At least with AMD's Zen cores the standard and C versions are very similar. The main changes are in caches and silicon density (which impacts max clock speeds). This design gives some big cores for the workloads that need it, but keeps everything else compact. Developers will just need to know to push the highest loads onto the bigger cores.
Perfect multi-threading is never going to be a thing for games, so this kind of design makes sense. In a sense, using an all-big-core design would be a waste.
Yeah that's my point though. Puts more on the developers plate. Makes it more difficult so hopefully whoever uses this rumoured chip develops/maintains good tools and libraries to make it easier on game development.