The logic behind the vehicle volume and payload expectation is the following:
An S1203 Transport/Minibus can hold 8 persons = 600 kg + trunk of 350 kg = 950 kg total.
An S1203 Cargobus can hold 2 persons = 150 kg + trunk of 800 = 950 kg.
Naturally the weight limits cannot shift depending how many people are in it. Just get the van you need or work with what you have.

kO_realisticTrunks

Changes vehicle trunk sizes in accordance with the kO_realisticItemSizes mod standard.
E.g. : A HMMV had 250 liter trunk, now it has about 680-860 liters trunk size.

kO_realisticTrunks

1.0.6

1.0.5

1.0.4

1.0.3

1.0.2

1.0.1

1.0.0