Property From A Custom UserControl That Is Saved In Viewstate Loses Its Value?

I have a custom control in an aspx page that has a property named Size

public int Size
{
get { return Convert.ToInt32(ViewState["CreativeSize"]); }
set { ViewState["CreativeSize"] = value; }
}

This property is set in the aspx page to a value lets say 500 during a postback called by a Button control that is in the page (not in the custom control).

I also have a button inside the custom control that raises a postback like this

protected void btnUpload_Click(object sender, EventArgs e)
{
if (fuBannerfile.HasFile)
if (fuBannerfile.FileContent.Length / 1024 > this.Size)
;//code here not important
}

When this event is called the "this.Size" property is 0. I also noticed during debugging that during Page_Load of the page, if i access the property like this:

int size = customControlId.Size;

the property is set to 500. But after that when the debugger reaches the event inside the control the property is 0.