The slots and stacks system doesn't meaningfully relate to carry limit either in weight (which also exists in game anyway), nor to item bulk. You can carry 50 stone in one stack - basically a boulder many times your size - but you can't stack two fish because they're slightly different sizes??

[edit]For fishing at least[/edit] All of this could be solved with a fish basket container item - a viking era chilli bin. 10 slots minimum, enough for quality 1 though 5 of two fish species at a time.