PrivateMemorySize64 will return the allocated memory in Bytes for a given process. Combined with LINQ its quite easy to find the top 5 memory eaters:

using System;
using System.Linq;

namespace ConsTopFiveMemory
{
   internal class Program
   {
      private static void Main(string[] args)
      {
         var query = (from p in System.Diagnostics.Process.GetProcesses()
                      orderby p.PrivateMemorySize64 descending
                      select p)
                          .Skip(0)
                          .Take(5)
                          .ToList();
         foreach (var item in query)
         {
            Console.WriteLine($"Process {item.ProcessName} with PID {item.Id}" +
               $" allocated  {item.PrivateMemorySize64} bytes of memory.");
         }
         Console.ReadLine();
      }
   }
}