If its too small, like that, your opponent gets confused by what it means.

"does he have a monster? or is he buffing and trying to steal the pot?"

This creates doubt about whether his hand is good or not, so he begins to respond passively, which is exactly what he did.

A normal bet says, I've got it or I'm representing. He either decides he's beat, or he thinks your bluffing and makes a move.