It is very safe to assume that micro-organisms function very differently in Fallout, since their natural world is different. We know radiation works differently in Fallout, and applying that knowledge to what we see in games, we can either say that micro-organisms were extremely weakened by radiation or that the developers did a poor job when thinking about how micro-organisms would be affected by airborne FEV and radiation.

It is possible that fallout radiation isn't as harmful to larger animals as irl. Maybe it's completely reversed in that very small organisms are extremely damaged, and large organisms take only minor damage from radiation simply because their cells are better at cooperating to repair themselves? This might be something that a very simple organisms, of just a few types of cells with very specific roles, might not be able to master to the same extent a higher number of cells working together might (This is a bit of a leap). This would absolutely explain, along with the increase of mutations from Fallout radiation and airborne FEV, why so many animals are much larger than their pre-war counterparts. You either had to get bigger or go extinct. Which might also explain why there are so few species of animals. The species that weren't lucky enough to grow a shit ton in size in only 200 years, and instead suffered adverse effects on large scales due to radiation and FEV, went extinct, and the ones that managed it became much larger.

It would also explain why you mainly only see human ghouls. Most animals are too small to survive the radiation that would turn them into ghouls, and most animals that live with humans and turn into ghouls would be killed. A cow's udders falling off when squeezed wouldn't really be useful. That's assuming other animals can even be turned into ghouls. I don't know if this is possible in Fallout.

Of course some animals didn't grow much in size. Brahmin, dogs/wolves, humans who'd only had slight to no FEV exposure did not become much bigger (brahmin might have been exposed to FEV but it is contested. The wikis seem to state that they were not exposed to FEV, but the Fallout Bibles apparently state they were. Nevertheless it's safe to assume they were bred to not eat up too many resources and be useful for travelling while still being big enough for humans to have a good reason to eat them). I think it's possible that humans, with their resources and superior intelligence are better at surviving in post-war Fallout than most pre-war species. There also seems to be an abundance of radaway and rad-x even 200 years after the war. Understanding and having access to them would really save your species the troubles of risking extinction and having to evolve. Using them to keep your animals alive and healthy while also applying breeding methods would also mean that your animals could evolve at a rate and in a direction that you wish. Knowing to not drink contaminated water often, and instead opt for clean water when possible would absolutely be beneficial. 200 years after the bombs fell it's still not that hard to find food and clean water if you scavenge a little. And we know from Fallout 4 that larger settlements will absolutely use water purification techniques and then sell any left-over water. So humans would probably be the best suited pre-war species to survive and have only minor mutations and complications in a post-war Fallout world. Sorry if I really rambled on. I find this really interesting so I could probably sit all day researching and discussing Fallout.