There are some functions in Sketch, that aren't immediately obvious, not even documented at all. One of them is the ability to resize a layer from a specific side in the "Size" fields in the inspector. By default, layers are changed from the left (width) or the top (height) if you alter their dimensions. But there are four suffixes (letters, that you add to the value of the input fields), that let you control this default behavior, two per axis.
- c: Used in the width field, this letter resizes the layer from the center.
- r: Also for the width, this changes the layer from the right.
- m: Used in the height field, this letter resizes the layer from the vertical middle.
- b: Also for the height, this changes the layer from the bottom.
Let me show you all four in action in the following animated GIF (click to play):
Probably an even better way is to use the "Scale..." dialog from "Edit" in the menu bar (shortcut Cmd + K), where you can visually define the origin of the manipulation. But for a quick change the fields in the inspector are still superior.