diff --git a/OpenRA.Mods.Cnc/FileSystem/BigFile.cs b/OpenRA.Mods.Cnc/FileSystem/BigFile.cs
index f5d9f0bdf5..ff1dc8aa67 100644
--- a/OpenRA.Mods.Cnc/FileSystem/BigFile.cs
+++ b/OpenRA.Mods.Cnc/FileSystem/BigFile.cs
@@ -25,7 +25,7 @@ namespace OpenRA.Mods.Cnc.FileSystem
 			public string Name { get; }
 			public IEnumerable<string> Contents => index.Keys;
 
-			readonly Dictionary<string, Entry> index = new();
+			readonly Dictionary<string, Entry> index;
 			readonly Stream s;
 
 			public BigFile(Stream s, string filename)
@@ -48,6 +48,7 @@ namespace OpenRA.Mods.Cnc.FileSystem
 					// and we don't have to try seeking there since the entries typically start next in EA's .big files.
 					s.ReadUInt32();
 
+					index = new Dictionary<string, Entry>((int)entryCount);
 					for (var i = 0; i < entryCount; i++)
 					{
 						var entry = new Entry(s);
diff --git a/OpenRA.Mods.Cnc/FileSystem/MegFile.cs b/OpenRA.Mods.Cnc/FileSystem/MegFile.cs
index 2f444a05aa..477cfde595 100644
--- a/OpenRA.Mods.Cnc/FileSystem/MegFile.cs
+++ b/OpenRA.Mods.Cnc/FileSystem/MegFile.cs
@@ -51,7 +51,7 @@ namespace OpenRA.Mods.Cnc.FileSystem
 		{
 			readonly Stream s;
 
-			readonly Dictionary<string, (uint Offset, int Length)> contents = new();
+			readonly Dictionary<string, (uint Offset, int Length)> contents;
 
 			public MegFile(Stream s, string filename)
 			{
@@ -84,6 +84,7 @@ namespace OpenRA.Mods.Cnc.FileSystem
 					throw new Exception("File name table in .meg file inconsistent");
 
 				// Now we load each file entry and associated info
+				contents = new Dictionary<string, (uint Offset, int Length)>((int)numFiles);
 				for (var i = 0; i < numFiles; i++)
 				{
 					// Ignore flags, crc, index
@@ -94,6 +95,8 @@ namespace OpenRA.Mods.Cnc.FileSystem
 					contents[filenames[nameIndex]] = (offset, (int)size);
 				}
 
+				contents.TrimExcess();
+
 				if (s.Position != headerSize)
 					throw new Exception("Expected to be at data start offset");
 			}
diff --git a/OpenRA.Mods.Cnc/FileSystem/MixFile.cs b/OpenRA.Mods.Cnc/FileSystem/MixFile.cs
index 7e6d1e17a4..6f8da0ec28 100644
--- a/OpenRA.Mods.Cnc/FileSystem/MixFile.cs
+++ b/OpenRA.Mods.Cnc/FileSystem/MixFile.cs
@@ -66,8 +66,6 @@ namespace OpenRA.Mods.Cnc.FileSystem
 
 			Dictionary<string, PackageEntry> ParseIndex(Dictionary<uint, PackageEntry> entries, string[] globalFilenames)
 			{
-				var classicIndex = new Dictionary<string, PackageEntry>();
-				var crcIndex = new Dictionary<string, PackageEntry>();
 				var allPossibleFilenames = new HashSet<string>(globalFilenames);
 
 				// Try and find a local mix database
@@ -88,6 +86,9 @@ namespace OpenRA.Mods.Cnc.FileSystem
 					}
 				}
 
+				var classicIndex = new Dictionary<string, PackageEntry>(entries.Count);
+				var crcIndex = new Dictionary<string, PackageEntry>(entries.Count);
+
 				foreach (var filename in allPossibleFilenames)
 				{
 					var classicHash = PackageEntry.HashFilename(filename, PackageHashType.Classic);
@@ -106,6 +107,7 @@ namespace OpenRA.Mods.Cnc.FileSystem
 				if (unknown > 0)
 					Log.Write("debug", $"{Name}: failed to resolve filenames for {unknown} unknown hashes");
 
+				bestIndex.TrimExcess();
 				return bestIndex;
 			}
 
diff --git a/OpenRA.Mods.Cnc/FileSystem/Pak.cs b/OpenRA.Mods.Cnc/FileSystem/Pak.cs
index 2c2bc0ef8a..2d686091d2 100644
--- a/OpenRA.Mods.Cnc/FileSystem/Pak.cs
+++ b/OpenRA.Mods.Cnc/FileSystem/Pak.cs
@@ -50,12 +50,11 @@ namespace OpenRA.Mods.Cnc.FileSystem
 						var length = (next == 0 ? (uint)stream.Length : next) - offset;
 
 						// Ignore duplicate files
-						if (index.ContainsKey(file))
-							continue;
-
-						index.Add(file, new Entry { Offset = offset, Length = length, Filename = file });
-						offset = next;
+						if (index.TryAdd(file, new Entry { Offset = offset, Length = length, Filename = file }))
+							offset = next;
 					}
+
+					index.TrimExcess();
 				}
 				catch
 				{
diff --git a/OpenRA.Mods.Common/FileSystem/InstallShieldPackage.cs b/OpenRA.Mods.Common/FileSystem/InstallShieldPackage.cs
index 9fcd247091..e389bb0612 100644
--- a/OpenRA.Mods.Common/FileSystem/InstallShieldPackage.cs
+++ b/OpenRA.Mods.Common/FileSystem/InstallShieldPackage.cs
@@ -37,7 +37,7 @@ namespace OpenRA.Mods.Common.FileSystem
 			public string Name { get; }
 			public IEnumerable<string> Contents => index.Keys;
 
-			readonly Dictionary<string, Entry> index = new();
+			readonly Dictionary<string, Entry> index;
 			readonly Stream s;
 			readonly long dataStart = 255;
 
@@ -63,7 +63,8 @@ namespace OpenRA.Mods.Common.FileSystem
 					s.Position = tocAddress;
 
 					// Parse directories
-					var directories = new Dictionary<string, uint>();
+					var directories = new Dictionary<string, uint>(dirCount);
+					var totalFileCount = 0;
 					for (var i = 0; i < dirCount; i++)
 					{
 						// Parse directory header
@@ -75,12 +76,16 @@ namespace OpenRA.Mods.Common.FileSystem
 						// Skip to the end of the chunk
 						s.Position += chunkSize - nameLength - 6;
 						directories.Add(dirName, fileCount);
+						totalFileCount += fileCount;
 					}
 
 					// Parse files
+					index = new Dictionary<string, Entry>(totalFileCount);
 					foreach (var dir in directories)
 						for (var i = 0; i < dir.Value; i++)
 							ParseFile(dir.Key);
+
+					index.TrimExcess();
 				}
 				catch
 				{
diff --git a/OpenRA.Mods.D2k/PackageLoaders/D2kSoundResources.cs b/OpenRA.Mods.D2k/PackageLoaders/D2kSoundResources.cs
index 878499d429..44cf1aae35 100644
--- a/OpenRA.Mods.D2k/PackageLoaders/D2kSoundResources.cs
+++ b/OpenRA.Mods.D2k/PackageLoaders/D2kSoundResources.cs
@@ -54,6 +54,8 @@ namespace OpenRA.Mods.D2k.PackageLoaders
 						var length = s.ReadUInt32();
 						index.Add(name, new Entry(offset, length));
 					}
+
+					index.TrimExcess();
 				}
 				catch
 				{