If you've got a world-changing, hugely scalable, easily copyable concept, then yeah, probably don't post it here. But otherwise, it's all in the implementation, and posting it on HN probably won't change that.
You want to be educating people of what you are doing. Copying an idea has little to do with the codebase being either small or clonable and more with the people behind the idea. And I don't mean just about having courage. Ideas by themselves are roughly worthless. There's no market for them. There's no place where one can go and buy an idea.
Describing your idea in detail doesn't mean other people will copy it. First they'll have to be convinced it's a good idea. If you ever tried to change anyone else's mind you know by now how hard that is. Not even founders themselves can predict how well their own ideas will do. Larry and Sergey originally tried to sell Google to Yahoo for $1m.
And even if people are convinced your idea is a good idea, they'll still have to compare it to the existing idea they are already working on and see which one they're more likely to do well with. A better, more ambitious idea might seem frightening. A simpler idea might seem more tangible. It could be at least a year before one can convince themselves it's ok to let an old idea die, and at least two years to pursue an ambitious one. Ambitious ideas really are that frightening.
If you are not convinced choosing between two ideas like this is hard, here's a simpler test that doesn't even involve a good idea. When you have only a bad idea and no good ones, how long does it take you to stop working on it?
Regardless, good ideas will have competition anyway. You can't avoid it. So actively working on the next step of getting feedback on what you have is a sign you are strong enough to take the next steps, however small they seem, as opposed to hiding to avoid competition.
Dropbox launched on HN (https://news.ycombinator.com/item?id=8863) and their biggest gain wasn't the number of users they got from HN. Their biggest gain was probably that they became less frightened by the idea of one day evolving into a startup with 300m users.
Ideas by themselves don't mean anything.. Its the execution of this idea that makes for a good venture. I personally meet so many people who don't speak or talk about their idea because they are afraid it will be copied. It just stops them from getting help from others.
On the other hand I have been open about our startup idea and what we are developing, and an amazing thing it has done for us, is the feedback it has gotten us and more importantly connections to right places and people.