diff --git a/Source/Core/DiscIO/CISOBlob.cpp b/Source/Core/DiscIO/CISOBlob.cpp
index fa9b30e2f5..f9947d0e6c 100644
--- a/Source/Core/DiscIO/CISOBlob.cpp
+++ b/Source/Core/DiscIO/CISOBlob.cpp
@@ -44,7 +44,7 @@ CISOFileReader* CISOFileReader::Create(const std::string& filename)
 
 u64 CISOFileReader::GetDataSize() const
 {
-	return GetRawSize();
+	return CISO_MAP_SIZE * m_block_size;
 }
 
 u64 CISOFileReader::GetRawSize() const
diff --git a/Source/Core/DiscIO/CISOBlob.h b/Source/Core/DiscIO/CISOBlob.h
index a1b5ae5cb7..dd0095f5a4 100644
--- a/Source/Core/DiscIO/CISOBlob.h
+++ b/Source/Core/DiscIO/CISOBlob.h
@@ -37,7 +37,11 @@ public:
 	static CISOFileReader* Create(const std::string& filename);
 
 	BlobType GetBlobType() const override { return BlobType::CISO; }
+
+	// The CISO format does not save the original file size.
+	// This function returns an upper bound.
 	u64 GetDataSize() const override;
+
 	u64 GetRawSize() const override;
 	bool Read(u64 offset, u64 nbytes, u8* out_ptr) override;