Do you have a source, both ways seem to have some logic to them (a larger growth is a smaller percentage growth for a large tree, for example - adding 1% to a massive tree is better than 10% to a tiny tree), but also we're used to new life increasing size far faster ...

Actually, I might tree rings not getting smaller as a tree grows taller is probably enough evidence to see that the carbon added increases over time? Would be nice to have a rigorous source to review too?