I have got my navel pierced about almost a year ago and still is not healed it does not hurt but is still.. how come? Is it because I'm bigger?

How can you tell that it is not healed? What exactly is it doing to make you think that?

It could be jewelry irratation, or when your sleeping its getting snagged and irratating it.

You could very well have an allergy to the ring in there--I would go back to the surgical steel post if you have not already.
My ears never healed for 2 years until I went to all surgical steel posts.