If it's honey and you want it thinner, get a sharp knife and cut it down...
"...if the space is insufficient, the bees shorten the cells on the side of one comb, thus rendering that side useless; and if placed more than the usual width, it requires a greater amount of bees to cover the brood, as also to raise the temperature to the proper degree for building comb, Second, when the combs are too widely spaced, the bees while refilling them with stores, lengthen the cells and thus make the comb thick and irregular--the application of the knife is then the only remedy to reduce them to proper thickness."--J.S. Harbison, The bee-keeper's directory pg 32