I get that, but they are also not "about the same size" I'm trying to have a learning moment here, so that to me is confusing the issue. I think that differing sizes are nice together. Although I think large size discrepancies may be too much.
I'm sorry to have confused you.
While the BIB trees are different sizes, they're between 8 and 18 inches.
Not 8 and 30 inches.
A three point display is for mid sized trees. Generally, they're larger than Shohin, but smaller than trees large enough to be shown on their own. The two trees are both "feature" trees, but since they're smaller, there's just not enough visual mass. The two trees plus an accent complete the three point "triangle". (Note: a scroll is sometimes used instead of an accent plant for the third "point".)
You can show a large tree with a smaller tree (Shohin) as the accent, but in this case, it assumes the role of "accent plant". The focus should remain on the primary tree. The accent tree is there to help show seasonality, movement, etc.
In a Shohin display, the trees in the display should be all about the same size. If one is significantly smaller than the others, it looks out of place.