Barring the cases wherein honey lovers may not give sufficient time to their slices of bread, the process which takes place is rather simple.
Water constitutes nearly 40% of apparently solid fresh bread whereas liquid honey has only 20% water against the denser 80% nectar which comprises of solution of glucose, fructose and sucrose secreted by most flowering plants. Thus water being lighter solution from the point of view of chemistry, diffuses in honey which is much more concentrated solution; through a natural process known as osmosis. As the diffusion of water content on the surface of the slice of bread in contact with honey starts, the slice of bread starts shrinking and taking concave shape. If honey is applied over the buttered side of slice, the water content of bread will not permeate in honey and the slice will not shrink to take concave shape.