XP Difference doesn't update for a TweenSize?

It works for the first level, but the XPDiff doesn't update, even in the loop. So eventually the bar will go over the intended size. Like so: https://gyazo.com/2ef5b5afad60a410f0a47abcd63a671a

Any help?

local starterXP = 1200
    while wait() do
        local XPDiff = Player.PlayerStats.XP.Value/starterXP*Player.PlayerStats.Level.Value
        script.Parent.PlayerInfoUI.CharFrame.InfoFrame.XPBar.Bar:TweenSize(UDim2.new(XPDiff,0,1,0), 'Out', 'Quad', .1)
        script.Parent.XPRankStat.XPBar.Bar:TweenSize(UDim2.new(XPDiff,0,1,0), 'Out', 'Quad', .1)