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Large bubbles like yours are one of the most common problems in fusing.  They happen when you fire too fast from around 1200F/650C to your top fusing temperature.  What happens is that the glass isn’t heavy enough to push out the air that’s trapped beneath the kiln shelf and the bottom of your piece.  If your kiln shelf isn’t level and flat it can be worse, but it can happen even with perfectly good shelves.

This kind of problem is most likely when you have less than two layers of glass (1/4″/6mm).  It can even happen with two layers if your schedule is too fast.

Fortunately, there’s an easy solution.  Make sure you have a bubble hold around 1230F/665C.  Hold for at least 30 minutes. Then go slower from the bubble hold to your top temperature.  Don’t go “as fast as possible.”  Slow down to 200F/110C dph or less.  The combination of the bubble hold and the slower ramp will help prevent that huge bubble.