Block inserting scale using mouse went too dramatic
The scale factor, when picked on-screen like that, is the distance in drawing units from the insertion point to the cursor location. If you define the tree Block so that at an Insertion scale of 1, its radius is 1 drawing unit (not a typical actual size of a tree), that won't happen.
One way:
Double-click on one of the Blocks, and go into the Block editor [it will trigger the BEDIT command], accepting the Block name it's offering [that of the one you double-clicked on].
The coordinate location 0,0 within the editing will be the insertion point of the Block. Start a SCALE command, select everything, give it 0,0 as the base point, call for the Reference option. For the start of the reference distance type in 0,0, and for the second point, pick anywhere on a perimeter Arc of the contents. For the new distance, type 1. Close the Block Editor, saving the changes.
That will change the size of any such Blocks already Inserted, so you'll need to Scale them up. You can pick one, and in the Properties palette, give it scale factors equal to the number of drawing units of the desired radius for that Insertion. If you want more than one at the same size, you can pick them all, and assign those scale factors in Properties to all of them at once.
