Weight is usually the answer but not the only one (especially if you're thin

).
Ask her if you tend to jump/move a lot in bed and if you seem to stop breathing sometimes. If so you might have sleep apnoea and you need to see your GP.
This website has some good info on snoring
http://www.britishsnoring.co.uk/
And I'd just like to add, don't underestimate the effect snoring has on your partner. If severe it can drive someone litterally crazy. If there's anything you can do to stop or at least reduce the snoring, do it not only for you but for your partner. Telling her to use ear plugs is likely to p*** her off (it did me).