What happens with mii is that you have a choice, you can either use 'device miibus' in which case you get all of the various drivers and the infrastructure, or you can use 'device mii', 'device rlphy', etc. if you wish to take a more fine-grained approach. Similarly, there is a miibus.ko that just depends on all the sub-drivers so you can still do 'kldload miibus.ko' to load all of them. I think this is probably a fine model as it will still load everything by default, but people who care about the space savings can trim things down as desired.