Why not? RSL are supposed to lay a large to extra large egg. They are the industry standard brown egg layer for eggs you see in the store. In large laying houses they control the final egg size by limiting certain proteins in the feed when the bird is still laying pullet eggs. We can't do that at home, so RSL often lay very large eggs in backyard chicken houses.
Does your bird that lays smaller eggs have the same basic coloring as this hen? If not, she's not a red sex link...if she is, you got a bummer bird or something. My rsl always lay huge eggs that didn't fit in the carton.