When we are creating new islands which are completely blank (i.e. no one built on them yet) we manually add a scaling factor to the forests which makes trees on the inside of tree clusters larger and on the outside of tree clusters smaller to have a transition that feels natural. Now for the tree painter we had to choose an average value that we deemed fitting and what we have now is what we ended up with.
I know this can look a little awkward especially on the screenshot provided in the forum post when you plant your custom trees next to trees that we have scaled quite large due to them being in the center of a tree cluster.
The models that are being used for the tree painter are indeed mostly the trees we used for creating the forests on the islands but the custom forests you can create are overall toned down a little bit in color variations, dead tree chance etc to let you create nice lush forests.
I hope that clarifies a bit :)