What are the factors that affect the amount of diffusion in gel electrophoresis?

molecular size, pore size

Smiling is caused by the electric field being uneven. This is influenced by the voltage you run the gel, the electrodes can have edge effects then. But uneven gel density and the buffer not controlling the temperature can also cause smiling. TAE buffer conducts higher electric current than TBE buffer, which will generate more heat and result in poor gel banding patterns. Excessive amounts of DNA/well will cause band resolution problems also. Never go for more than about 50ng/band from a 3mm wide well. But if the band gets less than 2 ng it will not be visible.