I do not understand why the finger deforms very little with respect to the su bone.
has the maximum weight, it should move along with your bone.
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The fact that it is red means that the influence of the selected bone on this part is the most it can be, but it doesn't mean that other bones don't have influence as well...
To make sure that this bone is the only one that controls this part of your mesh you can select it in Edit mode and in the Properties panel > Object Data > Vertex Group, on the dropdown menu on the right of the vertex grouo list (down arrow button), choose Remove From All Groups, then reassign to the right group with the Assign button.