I used ViewStatePicker in FireFox to analyse content of the viewstate.

First surprise : keywords and description are included in viewstate!
I just added enableviewstate="false" in root.master and it solves this problem.
<head id="Head1" runat="server" enableviewstate="false">



Another point : on the home page, topicHomePageText is also in the view state.
I have added enableviewstate="false" in default.aspx
<nopCommerce:Topic ID="topicHomePageText" runat="server" TopicName="HomePageText"
        OverrideSEO="false" EnableViewState="false"></nopCommerce:Topic>

and change the folowing code to comment IsPostBack condition
protected void Page_Load(object sender, EventArgs e)
{
    //if (!Page.IsPostBack)
    //{
        BindData();
    //}
}