I mostly play adventurer mode, so here are my two cents from that experience:
1. Effective Strength seems to be both VERY dependent on the size difference and independent from it based on what you are trying to do. The tiniest, scrawniest dwarf adventurer can achieve "Superdwarven" strength to easily hold mountains of loot without being slowed down, collapse heads with the punch, lop off giant tiger heads with the axe or spear the brains of the megabeasts with ease. However, even the strongest biggest meanest Superdwarf will never be able to wrestle the smallest horse - all of your attempted joint locks and chokes will fail with the message "You adjust your grip of the horse's throat". The very same weakest horse can easily break said Superdwarf's spine in two if it decides to wrestle back, or trample him to dust if it decides to charge and connects. Same goes for any animal that is the size of or bigger than a cow. Cue the "strongest dwarf vs weakest horse" gigachad meme.
2. Muscle mass mostly means thicker and heavier tissues. It does not affect the strength per se, BUT - every creature with large muscle mass you will encounter in vanilla game will usually be big enough to be the case for point 1 above. Thicker tissues also means slashing/piercing attacks are less effective, and blunt attacks can be outright useless. Go ahead, compare the amount of time it takes to kill a "tall muscular goblin" with a silver warhammer to the guts vs a "frail rhino". Also, charging the said "frail rhino" even as a "tall muscular" rhino man is a bad idea. HOWEVER, biting and shaking will fold that rhino's spine no problem regardless of its size, that is, if you can get a good latch before rhino breaks it (bites are counted as wrestling).
3. Deriving from the previous fact that Muscle mass does not affect the in-game strength per se, being swole in adv. mode can be a boon but has its trade-offs. Muscles have weight. Weight slows any unit carrying it down. So at some point the effective "thrust-to-weight" ratio of that unit will get pretty low. A muscular, tall starting adventurer will not be able to reach the sprint speeds of 4.000 and above, since at some point he will get too swole for it. In the past versions of the game (0.34 to 0.40 something) this "suffering from success" effect could be observed on zombies - prior to 0.44 zombification boosted the creature's strength and muscles TREMENDOUSLY. As in zombie chickens ripping apart giant lions and breaking elephant femurs with their pecks. This, however, also increased the creature's muscle mass so much that it was effectively immobile and anyone could just outwalk it. Zombified cheetah not catching up to a crawling goblin was a sight to behold.
Hence, points to take:
1. Size directly correlates with tissue thickness and affects the creature's wrestling power, charging power/resistance and max sprint speed and tissue thickness. It does NOT directly affect the strength of the weapon swings or unarmed blows.
2. Amount of muscles (""musucular description") does NOT directly affect the strength of the weapon swings or unarmed blows. It, however, correlates directly with Size and tissue thickness - bigger size = more meat on the bones between your blade and the organs. Hence more muscles = Thicker tissues = primarily bigger size relative to avg. = more wrestling power, slashing/piercing/blunt resistance and charging power/resistance
3. Thicker tissues have more weight than thinner tissues. Weight will slow that creature down. usually it is not too significant, but can be noticeable. Neither size nor tissue thickness will save the creature from spine fold from biting and shaking once the biter latches on firmly. Size WILL make breaking the bites from smaller creatures almost guaranteed though.
Rule of the thumb formula for decision making:
Bigger Size = Thicker tissues = More wrestling power, slashing/piercing/blunt resistance and charging power/resistance.