It's probably due to the "Output size" setup used in the graph nodes. Be sure to specify an "Absolute" size to your bitmap nodes, and a "Relative to parent" size to the node just after each one.
I've made a graph for a weapon with base materials and some weathering. When I import into Unity it allows me to change its resoltion but nothing changes really - it's the same 256 pixels. The graph uses baked occlusion, normal map and color map nodes. what can cause this problem? edit: also this old problem which I've…