Yeah, I know it can be mismatched sizes, the laptop i'm typing this on has 4gb soldered + a 16gb DIMM. My question was more trying to understand why manufacturers seem to prefer using one of each rather than just making both replaceable, since the hybrid approach makes it only partly upgradeable while taking up as much physical space as if both slots used removable DIMMs. Since it seems like this combines all of the disadvantages of fully replaceable and fully soldered RAM with only half of an advantage, why are there so many laptops which do it?