I believe you misunderstand what shrink and grow do. they shrink and grow path shapes. Here is a demo. I cloned an array of squares, unlinked the clones, then converted them all to paths (tweek tool only works on path objects).
For each, I stoked with my mouse around the outside and clicked a few times in the middle.
It has the same effect as insetting & outsetting, but localized in the area of the tool's "brush":
I am speculating you want something like this:
In either case, you will have to use Modify Paths->Add Nodes. Here I used a maximum of 15 px between pixels.