Why should they have been significantly thicker 500 years ago? My understanding is that, when oil paint is applied to a canvas and starts to cure, it expands for a while, and then contracts, before eventually drying and becoming pretty-much inert. Depending on the proportion of oil in the paint, and possibly the presence of other substances, the curing process can take anything from a few weeks to several months for a single layer of glaze or application of impasto. I've never come across anything that suggests that layers of glaze will continue to shrink over centuries.
Originally Posted by Mark Crabtree
The amount of expansion and contraction, at least with impasto, is tiny - imperceptible in the impasto stuff that I've done, so I'd assume the change in bulk/thickness is in the order of a few percent. My impasto brushstrokes, several years later, look exactly like they did when I applied the paint, so my feeling is that after the paint has expanded and shrunk, it returns to something very close to its original size.