Printing a three legged spacer in PETG that needed support so took the tree (auto) option. When viewed after slicing the supports are different diameters, two small and one large! The part being printed is symetrical around its centre line so why would BS make the supports different?
There are lots of search hits out there for organic supports being asymmetric for otherwise symmetric-appearing parts for all modern slicers in the Slic3r family; this is because everyone, afaik, is using the same core algorithm for tree generation, albeit with adaptations.
Tree supports start by identifying overhangs and calculating what the contact geometry will look like, then they build backwards from there down to the build plate.
The key is when I said “appearing” - remember, when we bring a model into Studio it’s a STL or a 3MF - in either case, it’s a surface representation of the original model made of a bunch of triangles. Even though the model looks the same as the original, there are differences in how each of the overhangs was recreated with triangles; that’s enough that when the tree support algo ran, it came up with a different “best solution” for that one overhang.
If it really bothers you, try increasing the STL file export resolution, and you can also play with rotating the model a few degrees, that may be enough to cause them to be calculated differently.
Takes a rip
They’re, like, just trees, maaaaan. Have you ever seen to trees that were exactly the same, y’know? They’re like, all natural and stuff, maaaaan, and you can’t control nature.
exhales
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The trunks on the smaller trees look a bit weak actually… or maybe it’s the angle of the screenshot. Determining the exact reason for the difference will likely involve tracing some rather complicated code… 
Out of curiosity you could rotate the model on the plate and see if that changes anything (eg. the skinnier trees are pointing diagonally while the thicker is along the X axis).
To add to the mesh resolution idea, there’s also Quality → Precision → Resolution process setting which can be lowered to increase resolution. And FWIW the imported mesh can be examined by going into the “simplify model” tool (r-click on an object, IIRC) and enabling the wireframe view option.
-Max