what did i do wrong?

[tough monolith]

local startSize = UDim2.new(0.15, 0,0.075, 0)
local HoverSize = UDim2.new(0.175, 0,0.085, 0)

script.Parent.MouseEnter:Connect(function()
script.Parent:TweenSize(HoverSize,Enum.EasingDirection.out,Enum.EasingStyle.Sine,.25,true)
end)

script.Parent.MouseLeave:Connect(function()
script.Parent:TweenSize(startSize,Enum.EasingDirection.out,Enum.EasingStyle.Sine,.25,true)
end)

[cosmic mist]

you shouldnt use GuiObject:TweenSize since its deprecated and no longer intended to be used

use TweenService instead