What Idiom can we use in this circumstance?
I ask someone for a little help but I get more than what I anticipate.
The main one that comes to mind is to "go above and beyond", which Cambridge dictionary gives as
to do more or better than would usually be expected of someone
If you wanted to thank someone directly, you could say:
Thanks very much for your help last week. I just wanted someone to pick me up from the hospital, but you got me some groceries and then stayed and made dinner for me too! That was really above and beyond.
You could also use it in the 3rd person:
I asked Peter for some recommendations on where to go in Paris and he gave me a whole list of recommendations and tips — he really went above and beyond.
Note that this would be a positive phrase: I'm assuming you're welcoming the extra information/help!