Q: In the sentence “Oni ne povas vidi ion pli malgrandan ol sablero”, why doesn’t “sablero” need the accusative ending -n?

A: It’s about the meaning of the sentence. Think about them this way:

1. Mi manĝas legomojn pli ofte ol [mi manĝas] viandon.

2. Mi vidas ion pli malgrandan ol [estas] sablero.

In sentence #1 you need the accusative on “viandon” because it’s the direct object of an understood “mi manĝas”.

In sentence #2, you don’t use the accusative on “sablero” because it’s not a direct object of anything. You’re not seeing a grain of sand. You’re seeing “ion”, and “pli malgrandan ol sablero” simply describes that thing. You need the accusative on “malgrandan” because it’s an adjective describing “ion”.

Very often in Esperanto, “unseen” grammar like this example comes into play. One example that everyone knows is the common “Bonan matenon!” Although we don’t think about it, the basis of this greeting is “[Mi deziras al vi] bonan matenon.”