Make ScaleTransform start from Center instead of Top-Left Corner

You can set RenderTransformOrigin to "0.5, 0.5"

<Style x:Key="sizeButton" TargetType="Button">
    <Setter Property="RenderTransformOrigin" Value="0.5, 0.5"/>
    <Style.Triggers>
        <Trigger Property="IsMouseOver" Value="True">
            <Setter Property="RenderTransform">
                <Setter.Value>
                    <ScaleTransform ScaleX="1.5" ScaleY="1.5"/>
                </Setter.Value>
            </Setter>
        </Trigger>
    </Style.Triggers>
</Style>