Rodents of unusual sizes? I don't think they exist.
I can only assume that Wesley stated this because their size isn't unusual. So let's talk rodents.
I'm sure you all know about the capybara. I mean, as I write this post on my phone, even the iPhone suggestions know about the capybara. Blah blah world's largest living rodent blah can weigh up to 150 pounds blah blah blah preferred prey of everything from the jaguar to the harpy eagle to the anaconda.
So let's talk about the coypu. You may know it as the nutria, or more colloquially the river rat. Sure, they only reach weights of 20 pounds or so, but these guys have a big presence. You can tell it isn't a muskrat because the coypu is about five times the size of their semi-aquatic cousin and lacks the laterally flattened tail. It's also smaller than a beaver with a distinctive rat-like tail.
So here's the thing. These started out in South America, but because of fur farms that were not as successful as they were hoped to be (people didn't want rat-fur coats, who would have guessed?), they've been introduced to North America, Europe, Asia, and Africa. And are they ever invasive. A coypu consumes 25% of its body weight daily, year round. They also go for the roots, killing off the entire plant before moving on. Combined with other factors (fires, floods, storms), coypu are helping destroy wetlands the world over. In fact, the state of Louisiana has a standing bounty on coypu tails of $5, seeing an average of 304,000 tails turned in per year.
So remember, when you go venturing into the fire swamp, there's a popping sound before each flame spurt, the lightning sand looks different from everything else, and keep the tails.