diff --git a/src/util/system.cpp b/src/util/system.cpp
--- a/src/util/system.cpp
+++ b/src/util/system.cpp
@@ -1118,20 +1118,24 @@
     SetFilePointerEx(hFile, nFileSize, 0, FILE_BEGIN);
     SetEndOfFile(hFile);
 #elif defined(MAC_OSX)
-    // OSX specific version.
+    // OSX specific version
+    // NOTE: Contrary to other OS versions, the OSX version assumes that
+    // NOTE: offset is the size of the file.
     fstore_t fst;
     fst.fst_flags = F_ALLOCATECONTIG;
     fst.fst_posmode = F_PEOFPOSMODE;
     fst.fst_offset = 0;
-    fst.fst_length = (off_t)offset + length;
+    // mac os fst_length takes the number of free bytes to allocate,
+    // not the desired file size
+    fst.fst_length = length;
     fst.fst_bytesalloc = 0;
     if (fcntl(fileno(file), F_PREALLOCATE, &fst) == -1) {
         fst.fst_flags = F_ALLOCATEALL;
         fcntl(fileno(file), F_PREALLOCATE, &fst);
     }
-    ftruncate(fileno(file), fst.fst_length);
+    ftruncate(fileno(file), static_cast<off_t>(offset) + length);
 #elif defined(__linux__)
-    // Version using posix_fallocate.
+    // Version using posix_fallocate
     off_t nEndPos = (off_t)offset + length;
     posix_fallocate(fileno(file), 0, nEndPos);
 #else