Did you ever wonder why water, when poured on a flat surface, doesn't spread until it is so thin that you can no longer see it? The reason it doesn't is because of cohesion -- the attraction between molecules that holds a substance together.
Cohesion on a liquid's surface results in a film-like force called surface tension. This is the same force that causes water to bead up.
When glass is heated to a liquid state, it too falls subject to surface tension. With soda-lime glasses (i.e. most common art glass) at fusing temperatures the surface tension forces the liquid glass to a height of about 5 mm. That means that if the glass is thinner the surface tension will pull it into a smaller area. If the glass is thicker then it will spread to a larger area until the height is about 5 mm. As the temperature rises, cohesion -- and thus surface tension -- weaken and the glass will spread more.
Surface tension can be a useful force for the artist who understands it. Well known kiln-glass artist Bob Leatherbarrow uses surface tension to achieve is unique trademark "crackle" technique.