don't assume, that is why a lot of people picking the 20X 2.5 thinking it is more efficient than the 10X 4.5.just to be clear, the housing is 5" in diameter at the top. it takes 2.5" cartridges if using it as an actual RO housing. you can get housings that take 4.5" cartridges, like this one:
i don't have any issues with the linked housing, but i have plumbed a bypass to reduce reactor flow, and my largest tank is 80 system gallons. i can definitely see how a larger one could be useful for a much larger tank than my own.