One big idea?

 I have sometimes been a bit sceptical of the idea that bible books / passages always have one big idea and that this should be reflected in every sermon. 

I think it is sometimes legitimate to preach something that is obviously not the main point. 

But I think we can say at least this:

(1) Your sermon should ideally have some kind of unity and coherence

(2) It should also have a purpose which is more than filling time or even pointing out a few things of interest. There should be application - an intended transformation or responses. And see also (1).

If you find there are many interesting things that a bible reading says, I think it would certainly serve your preparation to think about any big ideas or aims of the book or section. Why does it say all this here? How does this fit in and fit together? This can help you to focus and to work towards coherence and purpose in your sermon. A view of the whole will likely illuminate the working of the parts. 

And thinking about the purpose or aim of the text for the first readers will help you to think about the aim(s) of your sermon - taking into account your hearers' different situation. It is conceivable that a warning to Old Testament readers could serve as a comfort to us, for example. God's word of salvation and judgement has proved to be true so we who are trusting in Jesus in the midst of suffering can be sure that vindication is coming.