If you center a resizable element and expand it left/right, it has the illusion that it is only expanding half of the mouse movement.
I don't know how you center your resizable but this test case works well for me.
At it's heart I do something similar to vol7ron but I use one assignment less and the calculation is better readable :D just joking choose what you like better fits you more
ui.size.width += ui.size.width - ui.originalSize.width;