Mens brain are larger than womens and contains more neurons (Pakkenberg et al). If you correct for body weight, rats should be much smart than human. Brain size is not a good parameter. E.g. the bump whales brain is about 7kg, but the whale is probably not smarter than humans.
fMRI has a time resolution about 1s. An action potential in the brain can get from any point to any other in about 30ms, so fMRI makes images on an unphysiological time resolution.
There is no reason to try to blur differences. However, there should be more studies into factors influenicing brain size, e.g. genes, hormones, physical training and other activities. E.g. we know that size of the sensory area for the arm get smaller if you anesthezise the arm just for a few hour. The brain is very dynamic.