rim size against weight, why.

Only suggestion I could throw up is the increase in unsprung weight of the 16 inch wheels. Unsprung plus sprung weight plus tare weight = gross vehicle weight. But it could only be the difference between the weight of the tyre/wheel assembly 15 verses 16 inch. I would not think we would be talking much more than 10 kg. total?

I'll hazard a guess, but it is only a guess...

If you maintain the same rolling diameter with the 16" rims as you have with the 15" rims, you will have lower profile tyres. I expect that the lower profile tyres will have a decreased load rating, and hence reduce the load rating of the axle.

Cheers
John

The load index of most 16inch tyres is greater than the load index of 15 inch tyres. I'd suggest that your source is incorrect.

In my experience, a bigger tyre not rim will carry a load better.

Changing the rim size will not effect the load rate as the rate is governed by the build of the van and spring capacity. As for towing weight, the different size will not effect anything aside from the height. In my opinion, I would go for a wider tyre if it is a major concern, it will carry the weight better.