It's perfectly normal. The blend bush works by analysing what is in the area of the brush and averaging the heights, the smaller the brush the less differences in the height in the area of the brush there are, so the less effect will occur, eventually diminishing returns means that there will be no effect because everything in range of the brush is almost the same height.
Primary Desktop:
i7 7700,k NV1070 8GB, 16GB 3200mhz memory, 1x 2TB Hybrid, Win10.
Secondary Desktop:
i5 4760k, NV960 2GB, 16GB 2333mhz memory, 1x 2TB Hybrid, Win10.
Primary Laptop:
i5, NV1050 4GB, 8GB memory, 1x 1TB HDD, Win10.
Secondary Laptop:
i3, Intel 4000 series graphics, 6GB memory, 1x 500gb HDD, Win8.1.
