Double clicking on the arrow of the right side of a group entry opens the subgroups. My experience with the arrow is that it is too small. Please make this arrow double size!
You can actually double-click anywhere on the group to expand it. The arrow is just a visual indicator not a button. Do you still think it is too small?
I just pushed a change to the latest development version so that I single click on the arrow toggles the expansion status. With this change, the behavior should be consistent with other programs (e.g. windows explorer). As the “hit box” is a bit larger than the actual arrow, I think the size should be sufficient.