I know I'm a bit late and this is going to be a little lengthy, apologies. Equip load works in percentages(as you already know), but let's look at your example further. You changed from a 5.0 weapon with 26.7 equip load and swapped to an 8.0 weapon and it increased to 33.9 equip load. You said you started off with a max load of 58 and bumped it up to 60 to accommodate for the new weapon's weight. Basically, Dark Soul's equip load works with diminishing returns and "balancing". So even though the new weapon should've been taken care of by the max load increase, the game instead calculated it needed to weigh more for it to be balanced. It's lame, but it's the game.