Help to understand a sentence from "The lumber room"

Hello! Could anybody help me to understand a sentence from «The lumber room» (by Saki).

The passage is : «Only that morning he had refused to eat his wholesome bread-and-milk on the seemingly

 frivolous ground that there was a frog in it.«….» Older and wiser and better people had told him that there 

could not possibly be a frog in his bread-and-milk and that he was not to talk nonsense. «…». The dramatic part of the 

incident was that there really was a frog in Nicholas' basin of bread-and-milk; he had put it there himself, so he felt entitled

 to know something about it. The sin of taking a frog from the garden and putting it into a bowl of wholesome

 bread-and-milk was enlarged on at great length….»

I can’t understand the last sentence. Who did enlarge on the sin? Does it mean that Nicholas told about his sin himself or this sentence implies that «older and wiser and better people» were discussing his sin in great detail?



Thanks in advance