As Minnakht explained the SIZE is based on what VOLUME the creature will be, it's up to your imagination to determine if that is tall, short and extreme fat, or the right amount of height + width + depth.
There are also Size Modifiers HEIGHT, BROADNESS that slightly alter the SIZE mass (so SIZE:70000 is the base, but with a lower HEIGHT and BROADNESS they could be 68000 or maybe 75000)
But the Tissues they are made out of, which also is the material makes up their weight. Which totals their Mass so a Bronze Colossus that is shrunk to the size of 70,000 (human size) would be able to hit harder than a human of the same size, simply because it can throw more mass around (Bronze is heavier in that amount of volume than the same amount of skin,fat,muscle, and bone.) and this is not factoring in STRENGTH to the punches which just says they can toss that mass around with greater force.
So even a Creature that is 30,000 fights a 60,000 creature if Creature A (30,000) has an incredible STRENGTH and TOUGHNESS stats (5000) they could potentially be able to go toe-to-toe with Creature B (60,000) if it is avg, Creature A will most likely lose still just because of it's smaller stature, which means less mass, and lower amounts of tissue thickness (bones will break easier, less muscle to absorb impact) only way Creature A would stand a chance is if all the materials it's tissues are made of are altered to be as strong as Creature B's materials scaled down.
It then goes into a deeper level when you are using bodyparts as each part has a relative size which I'm not sure if they try to restrain themselves within the given SIZE volume (70,000) as upper bodies are a RELSIZE of 1000, and a thigh is 900
And that is just the body parts, like external limbs so the total of all RELSIZEs for a humanoid is 5450 (total of external parts) and this is excluding organs
And these parts grow or shrink depending on the Size of the creature they are apart of, as well as the tissue thicknesses
As tissue is calculated something like Ratio/avg math Putnam has the formula but it's roughly
Tissue Mass/volume/layer thickness = BP RELSIZE * (Tissue Thickness / (sum of all Tissue Thicknesses layering part))
I suck at math so don't take what I just posted for a formula to heart but it gets crazy complex working the numbers.