Reading: Difference between 50M vs 25M vs 16M bitrates
If you are shooting a static subject with no fine detail, the smaller bit rate is probably enough to capture all the information, and there is no visible difference between the bit rates.
Bit rate setting typically specifies the upper limit for the actual bit rate. In case of static subject, the actual bit rate can be than the maximum bit rate. Try shooting for example foliage when there is some wind. You should start seeing differences in the image quality and probably also in the file size.