When in the kiln on a full fuse programme, glass will naturally want to get to a 6mm thickness.
If you're trying to work with less glass ie using powdered glass or thinner sheets or stacks of glass ie less than 6mm, it will pull in from the sides during firing giving a 'dog-bone' effect.
If you've stacked glass sheets, added powders, frits or other glass accessories that might make the thickness over 6mm, the glass will spread out during full fusing so that it came achieve 6mm thickness.
This 'rule' seems so easy to remember but when you're adding little bits of glass or trying to achieve a particular design, it's so easy to forget and then open the kiln lid to discover that the glass fairies have pulled in or splurged out the glass. Very frustrating when you've spent an age putting a piece together but definitely the first rule that you need to know AND remember.
(diagram from http://glasstips.blogspot.com/2016/06/dog-boning-causes.html)