Strength of bump depends of object scale? Look at the Blend file:



On frame 2 we have an cube with bump texture:



On Frame 1 we have same object, same material and same lightning (one sun). The only difference that this cube scaled 500 times:



As you noted the bump map on scaled object affects so small, that faсes looks flat.

Same result on different bump methods. First, Using displacement (not real geometry, experimental features turned off):  



Second, with bump node:



Is it possible to get rid of this problem?
I think, it's a limitation of bump method. You can noted, that strength of bump effect not depends of scale, it depends of how big current face in world space...

So, use normal map. Normal map uses angles instead of depth, that's why objects, textured with normal map will looks the same in all scales.



Related: https://blenderartists.org/forum/showthread.php?359453-Cycles-displacement-corruption-when-scaling-an-object