Linda Ethier – who has been kiln casting glass since the late 1960s and who appears in the orginal Glass Fusing Book #1 by Boyce Lundstrom and Dan Schwoerer has a great expression that covers this:
“There are certain laws in place when fusing glass. The bigger the piece, the more the laws are enforced.”
As work gets smaller you can “cheat” – but you need to understand what you are doing and why it will (or will not) work.