My wife has assured me that her bread doesn't have time to go concave when spread with honey. However, for those folk who chomp their honeyed bread in a more leisurely fashion, there is a simple explanation.
Bread is approximately 40 per cent water while honey is a strong solution containing approximately 80 per cent sugar. This means that moisture is drawn out of the bread and into the honey by osmosis. Removing the water makes the bread shrink, but only on the side exposed to the honey. This causes the bread to become concave.
This is less likely to happen, of course, if you butter your bread before
spreading the honey. Butter forms a water-impermeable layer that protects
the bread from dehydration by the honey.
Peter Bursztyn, Barrie, Ontario