Okay, so for the uninitiated, Drakar och Demoner (DoD) was a derivative of Chaosiums Magicworld which basically became the norm for roleplaying games in Sweden in the 80's. With the "Expert" expansion it moved to a D20 but remained very BRP-adjacent. If you ever wondered why Trudvang and the original Kult and Symbaroum are all D20 roll-unders, there you have it.
Anyways, encumbrance.
First, ENC values for a few assorted weapons. As mentioned, 1 ENC is meant to be roughly 3 kilo but weapons are rounded up quite a bit to account for "bulk" in carrying them.
Most standard swords, maces etc. are 1 ENC, an axe, spear, bastard sword 2 ENC, pole weapons 3 ENC.
Armor uses a SIZ table similar to RQ3, but for a SIZ 12 human, you'd find armor "weighing" f.x.
1.5 ENC for studded leather greaves / vambraces.
1 ENC for a roman helmet.
4 ENC for a mail shirt
2.5 ENC for metal chest plate.
6 ENC for a mail hauberk.
And so on.
In the basic rules, you can carry ENC equal to STR and thats it. No penalties, you just can't carry any more. So a STR 12 human could have a mail shirt, broadsword and helmet and still have 5 ENC left for torches, backpack, packed lunches and rubber ducky.
Expert added a "proper" encumbrance table.
Carrying up to 0.5 x STR = no penalties.
Carrying up to 1 x STR = -20% movement speed, -1 DEX
Carrying up to 1.5 x STR = -40% speed, -3 DEX
Carrying up to 2 x STR = -60% speed, -7 DEX (and dex skills are impossible).
The DEX penalty is applied to DEX based skills (D20 roll under, so -3 is equal to a -15% in BRP)
I hope that helps? It always seemed to be a decent balance because to an extent you can count "things" without being as simplistic as some slot systems.
If desired, I can dig out more numbers and such.