You don’t give the temperatures or schedule you are using for your slumping, but from your description I bet that you are using too high a temperature or too long a soak. You need to observe when your piece has slumped. This is done by frequent, but short peeks into the kiln to monitor the progress of the slump. You don’t indicate the nature of the mould – size, diameter, depth – so it is difficult to be precise.
If you are using deep moulds, the glass will become smaller as it moves to accommodate itself to the curve and depth.
There is quite a bit of information on my blog. Here is one that relates directly (in part) to your question: http://glasstips.blogspot.com/2010/04/slumping-unknown-glasses.html
You will find that I also am in the midst [most of April] of a number of tips on slumping if you go to the top of the blog: http://glasstips.blogspot.com/
If you really must have a piece of glass much larger than the mould, make a collar to go round the mould (which needs to be supported from below). This most often will be fibre board. This will support the glass while it softens. However if you have a lot of glass outside the mould rim, it will not move into the mould, but simply thin where it stretches down into the mould.