Items Changing Player Stats, Backpack increasing Inventory Size

I guess it can be done in many ways. For example you can start with having base backpack class:

 public abstract class Backpack
 {
     private int maximumWeight;
     private List<Item> items;
     
     public Backpack(List<Item> items = null)
     {
         this.items = new List<Item>(); 
         this.items = items;
     }
 }

The you can add three class (one per tier):

 public class SmallBackpack : Backpack
 {
     maximumWieght = 10;
 }
 
 public class NormalBackpack : Backpack
 {
     maximumWeight = 15;
 }
 
 public class BigBackpack : Backpack
 {
     maximumWeight = 20;
 }

Some class containing information about player should have field and a method:

 private Backpack playerBackpack;

 public void ChangeBackpack(BackpackType newBackpackType)
 {
     switch(newBackpackType)
     {
         case(small):
             playerBackpack = new SmallBackpack(playerBackpack.items); //it will move current items to new backpack
             break;
         case(normal):
             playerBackpack = new NormalBackpack(playerBackpack.items);
             break;
         case(big):
             playerBackpack = new BigBackpack(playerBackpack.items);
             break;
     }
 }

After all another class, for example WorldItem can call Player.ChangeBackpack(BackpackType).

I've missed some null checks, correctness and in my opinion WorldItem calling method on player is basically bad idea, but I hope you get the point :)