I thought Tim started looking taller than Dick back in Knightfall—at least in some panels. It was partly the effect of being more willowy. But Tim did a lot of growing up in the previous few years, then stayed the same or shrank under other artists so he could headline his own series about growing up.
It makes some sense for Tim eventually to have a couple of inches on Dick. Even if Dick does always seem to be drawn taller than Kory, he's officially only 5'10" (178 cm).