diff --git a/Lib/test/test_bz2.py b/Lib/test/test_bz2.py
index c97ed1cea0d..ba1c02c8338 100644
--- a/Lib/test/test_bz2.py
+++ b/Lib/test/test_bz2.py
@@ -721,10 +721,10 @@ class BZ2DecompressorTest(BaseTest):
     @bigmemtest(size=_4G + 100, memuse=3.3)
     def testDecompress4G(self, size):
         # "Test BZ2Decompressor.decompress() with >4GiB input"
-        blocksize = 10 * 1024 * 1024
+        blocksize = min(10 * 1024 * 1024, size)
         block = random.randbytes(blocksize)
         try:
-            data = block * (size // blocksize + 1)
+            data = block * ((size-1) // blocksize + 1)
             compressed = bz2.compress(data)
             bz2d = BZ2Decompressor()
             decompressed = bz2d.decompress(compressed)
diff --git a/Lib/test/test_lzma.py b/Lib/test/test_lzma.py
index 145c8cfced4..49042d7390b 100644
--- a/Lib/test/test_lzma.py
+++ b/Lib/test/test_lzma.py
@@ -352,10 +352,10 @@ class CompressorDecompressorTestCase(unittest.TestCase):
     @bigmemtest(size=_4G + 100, memuse=3)
     def test_decompressor_bigmem(self, size):
         lzd = LZMADecompressor()
-        blocksize = 10 * 1024 * 1024
+        blocksize = min(10 * 1024 * 1024, size)
         block = random.randbytes(blocksize)
         try:
-            input = block * (size // blocksize + 1)
+            input = block * ((size-1) // blocksize + 1)
             cdata = lzma.compress(input)
             ddata = lzd.decompress(cdata)
             self.assertEqual(ddata, input)