Interesting question. Clams, maybe?

As another response notes, dogs have a wide range because of selective breeding, a/k/a artificial selection. Dogs, amongst domesticated animals, are also distinguished in serving as service animals principally, rather than (widely at least) for meat, fur, feathers, milk, eggs, or other attributes. Draught animals (horses, donkeys, mules, oxen, water buffalo, llamas, and camels) would be somewhat similar in that regard, though dogs provide other services, with transport being a relatively limited role. (Others: hunting, vermin control, companionship, guard or sentry, managing herd animals, off the top of my head.). Food- and fur-bearing animals are typically selected for reasonably large sizes for yield maximisation.

So even among other service species the incentives to breed for

Then there's the question of what species we're considering: animals only, vertebrates, and/or mammals? I'm going to mostly stick with vertebrates.

Looking again at artificial selection, this involves two characteristics that are difficult to match in the wild: persistence of interbreeding, that is, even the most widely disparate sized do breeds can still mutually reproduce (though possibly by artificial insemination), and cultivation of different phenotypes, that is, a given breed typically is protected against unwanted crossings with others having differing characteristics. This combination is difficult to obtain in the wild, as interbreeding requires shared habitat whilst variant-cultivation requires exclusive breeding.

The species which might be best suited to this would probably be highly numerous, capable of surviving across a wide range of habitats, and see at least some chance for mating across their entire geographic distribution, and at the same time cultivating localised specialised populations favouring highly divergent sizes. That's ... a fairly tall order.

I'd expect that you'd most likely find that amongst rodents or grazing herd animals, where specific island populations (in the geographic and/or biological senses) emerge. Possibly amongst a species such as wild boar or pigs --- domesticated pigs see fairly high variance. I'd expect that most bird populations would tend to have fairly high interbreeding capabilities (though Darwin's Finches suggest otherwise). Salt-water fish in the open oceans would likely intercommunicate across populations to too great an extent, though individual isolated populations within lakes or caves might rapidly diverge.

I know that pygmyism and gigantistic traits have been seen among elephants, mammoths, hippos, and sloths, though I don't know if these were considered separate species. To take one example, giant sloths could way over 1,000 kg, whilst a typical sloth today weighs about 5 kg, for a 200:1 mass ratio, though probably of different species. Among giant sloths, full-grown adults ranged from ~2 ft (60 cm) to 20 ft (6 m), again among what we'd probably consider different species. (There are currently 6 recognised distinct sloth species.)

It's also worth keeping in mind that dogs have been domesticated for a relatively short time, about 40,000 years, and most dog breed development has occurred far more recently than that, with the oldest distinct breed dating back only 10,000 years. My understanding is that many of the breeds we're familiar with now date back only to the 19th century or so, see: <https://www.businessinsider.com/dog-breeds-victorian-england...>. This means that dogs have seen immensely fast divergence in morphology while preserving genetic compatibility. For species in the wild, you'd likely have to find examples of similar separation and selective differences over a comparable time period.

Outside of mammalia: I'd expect arthropods, shellfish, coral, or similar sealife to be the most likely candidates for widely-divergent sizes within a single species, just given sheer abundance of species. Giant clams range up to 200 kg, and grow continuously through their life, so you might find individuals with widely ranging sizes, though not necessarily different breeds or populations.