Use HeapAlloc to allocate hunk memory for the engine on Xbox. This reserves memory from the title's pool as opposed to the system's pool, which allows us to get the full 128 MB (or more) that we desire.

Assets/Scripts/QuakeParms.cs

@@ -49,7 +49,13 @@ public class QuakeParms
     public void AllocateMemory(int memorySize)
     {
         memSize = memorySize;
+        
+#if UNITY_GAMECORE
+        memBase = SystemLibrary.HeapAlloc(SystemLibrary.GetProcessHeap(), 0, new IntPtr(memorySize));
+#else
         memBase = Marshal.AllocHGlobal(memorySize);
+#endif
+        
         if (memBase == IntPtr.Zero)
         {
             throw new OutOfMemoryException($"Could not allocate {memorySize / 1024 / 1024} MB of heap memory!");
@@ -66,7 +72,11 @@ public class QuakeParms
         
         if (memBase != IntPtr.Zero)
         {
+#if UNITY_GAMECORE
+            SystemLibrary.HeapFree(SystemLibrary.GetProcessHeap(), 0, memBase);
+#else
             Marshal.FreeHGlobal(memBase);
+#endif
             memBase = IntPtr.Zero;
         }
 

Assets/Scripts/SystemLibrary.cs

@@ -53,4 +53,16 @@ public static class SystemLibrary
     [DllImport("SceKernelShim.prx")]
     public static extern IntPtr GetProcAddress(IntPtr hModule, string procedureName);
 #endif
+    
+#if UNITY_GAMECORE
+    [DllImport("kernel32", SetLastError = true)]
+    public static extern IntPtr GetProcessHeap();
+    
+    [DllImport("kernel32")]
+    public static extern IntPtr HeapAlloc(IntPtr hHeap, int dwFlags, IntPtr dwBytes);
+
+    [DllImport("kernel32")]
+    [return: MarshalAs(UnmanagedType.Bool)]
+    public static extern bool HeapFree(IntPtr hHeap, int dwFlags, IntPtr lpMem);
+#endif
 }

Assets/Scripts/UniQuake.cs

@@ -5,11 +5,7 @@ using UnityEngine;
 
 public partial class UniQuake: MonoBehaviour
 {
-#if UNITY_GAMECORE
-    private const int DefaultMemSize = 0x2000000;  // Xbox is limited to 32 MB of heap space for now...
-#else
     private const int DefaultMemSize = 0x8000000;  // 128 MB of heap space
-#endif
 
     public int memorySize = DefaultMemSize;