The size of the exit hole, as has been mentioned several times, will be determined by several factors. One that I haven't seen mentioned yet would be the velocity of the exiting bullet. If you're shooting a caliber and bullet combination that could be described as "over kill" on any animal, then one could/should expect larger exit holes than entry holes. For example, a .243 using 80 sp bullets on a coyote. We all know that there is going to be an exit on nearly every shot angle, and it would be expected that the exit hole is going to be large, for sure larger than the entry hole. Then look at the opposite end of the spectrum, maybe described as "under-gunned". I shot several elk with a .270 and of the dozen plus I only every had one exit, and most always found the bullet under the skin on the offside. BUT the one exit that I did get was from a 140 gr. fail safe and that exit, although larger than the entry, was not much larger because the bullet didn't have much energy left to blow a large exit hole.
Look at the extreme spectrum and think about what a .204 shooting a 32 or 39 gr. BK at about 3600-4000fps into a prairie dog, one can expect the exit hole to be astronimical, when compared to the entry hole!