In my world I have a type of magic user who grows stronger the more they kill animals and people.
Currently I conceptualise this as a sort of pocket dimension muscle, whenever they kill a creature, its muscle mass gets added to that pocket dimension. When the user wants to augment their strength, they can exert this 'magic muscle' instead.
For example, if they killed a 80kg man with 40% muscle mass, the magic user would gain about 32kgs (80 x 0.4) of muscle to use. This magic muscle matches whatever parts of the users body are being exerted. So when they throw a punch, that 32kgs of muscle is being divided up and added to their arms, shoulders, hips, legs.
The actual composition of the users body isn't changing, they're not growing larger/heavier, so leverage and bone density are still limitations on their abilities. The magic is acting like an exoskeleton, being exerted instead of their body, but matching its movements.
Additional information that might be important for people much smarter than me:
The story is set in an alt history south pacific (the Cook Islands, to be pacific specific), so in terms of animals they can gain strength from, they're limited to fish, dogs, pigs, birds, and the occasional whale.
The composition of this magic muscle is dependent on the animals its being taken from, so gaining extra fast twitch fibers for extra speed is possible, even if the original animals anatomy isn't analogous to a human (the muscles in a crayfish's tail could be used to augment the magic users arms, etc).
I'm aware of, and willing to handwave a small violation to the laws of physics regarding the 'gaining' of strength. Since the dead animals body mass isn't going anywhere or shrinking, the magic user is technically adding energy into the universe by copying the animal's body mass. For the pedants out there, let's say there's a distant star that is getting 0.00000000000001% dimmer to compensate for this.
How much stronger would these people be compared to the average person? What kind of feats could they pull off?
Additionally, are there certain animals they'd value killing over others, for reasons other than pure muscle mass? (Unique composition, high fast-twitch fiber ratio)