My coworker has a Toy Fox Terrier that is taller, longer (base of skull to base of tail), and bigger around than my smallest cat. But Pixie (the dog) weighs almost three pounds less than Cosmo (the cat).
Both animals are fit and very active. How does Cosmo weigh nearly seven pounds while Pixie weighs barely four?
I suspect that the differences my vary by breed, rather than species.
Years ago, we brought home a 12-week-old Boxer puppy to meet our 24-week-old Manx kitten. They were identical in height, length, and breadth, but the puppy weighed three times as much as the kitten. Boxers are dense, (well, they have a high weight to volume ratio). The something-or-other shepherd next door is about the same height, length, and breadth as our current Boxer and weighs about 1/3 less.
I don’t know, but it’s often very suprising how much thinner a dog gets if you shave it 
Cats, too. They don’t just get thinner, they get offended!
Wet cats look like skinny and ugly. My siamese had to be washed after sleeping under a truck sump. I wish I’d photographed him, he looked like a rat.
I’d say differences are by breed and individual. Some breeds are stockier (heavier bones) or carry more muscle (heavier tissue) than others, some individuals are as well.
I’ve got a basic DSH (domestic short hair) orange-tabby cat. He’s physically very small compared to other cats - I’d say about 2/3 normal cat size (length & height). He isn’t at all overweight, I’d say his build is average leaning towards stocky.
He weighs 12 pounds. It sometimes hurts when he stands on you, due to the amount of weight on each of his little paws.
As I usually say, he’s the densest cat in the world. Yes, that way too. And he has the cat-trick of making himself even heavier when he wants to. Then we call him the NEUTRON-CAT!
There’s a lot of variation betgween dog breeds. I remember when I was young, my family had a Sheltie and my neighbor had a Beagle. They were about the same length and they looked to be the same width, but the Beagle was much heavier.
I tried some experiments along this line, but the water volume displacement tests were irritating the hell out of me.