Raistlins brother (can't remember his name) had this problem in the Dragonlance books at the start of an adventure. One should also not overlook the ridicule factor for your rotund knight. (Though we are drifting a bit from the topic of familiars)
So.. In our games we also used the True Familiar spell, but you had to dind or research this, which gave access to the urh "True" familiar, basically a spirit entity in animal form as opposed to a very close releationship with a natural animal.