w3wp eating up memory and not giving it back

I have a classic asp.net page in .net 4.6.1. 

It loads 4 MB of data (they want it on one page) and no matter I simplify it, the IIS Worker Process w3wp.exe eats up a Gig of data, and never expires anything or gives any memory back.

Why?

<%@ Page Title="Home Page" Language="C#" MasterPageFile="~/Site.Master" AutoEventWireup="true" CodeFile="Default.aspx.cs" Inherits="_Default" %><asp:Content ID="BodyContent" ContentPlaceHolderID="MainContent" runat="server"><asp:GridView ID="gvSelectionList" runat="server" AutoGenerateColumns="false" CssClass="LPSCriteriaSelection" EnableViewState="False"><Columns><asp:TemplateField HeaderText="SerialNumber"><ItemTemplate><asp:HyperLink ID="hlBSN" Text='<%#GetSerialNumberText(Container.DataItem)%>' NavigateUrl='<%#GetSerialNumberText(Container.DataItem)%>' /></ItemTemplate></asp:TemplateField></Columns></asp:GridView></asp:Content>

    using System;
    using System.Collections.Generic;
    using System.Web.UI;

    public partial class _Default : Page
    {
        protected void Page_Load(object sender, EventArgs e)
        {
            if(this.IsPostBack) return;
            ExpandableSelections items = new ExpandableSelections();
            if(items.Count == 0) return;
            this.gvSelectionList.DataSource = items;
            this.gvSelectionList.DataBind();
        }
        protected string GetSerialNumberText(object dataItem)
        {
            SerialNumberData item = (SerialNumberData)dataItem;
            return item.SerialNumber;
        }
    }
    public class SerialNumberData
    {
        public string SerialNumber { get; set; }
        public SerialNumberData(string data) { SerialNumber = data; }
    }
    public class ExpandableSelections : List<SerialNumberData>
    {
        internal ExpandableSelections()
        {   // Emulate database call
            for (int i = 1; i < 72000; i++)
                this.Add(new SerialNumberData("XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX"));
        }
    }