diff --git a/include/QF/set.h b/include/QF/set.h
index 57c8342de..1d59ed395 100644
--- a/include/QF/set.h
+++ b/include/QF/set.h
@@ -162,6 +162,15 @@ void set_delete_r (set_pool_t *set_pool, set_t *set);
 */
 void set_expand (set_t *set, unsigned size);
 
+/** Shrink the set's backing memory to the minimum required to hold the set.
+
+	This does not affect (nor is affected by) whether the set is an infinite
+	set.
+
+	\param set      The set to trim
+*/
+void set_trim (set_t *set);
+
 /** Add an element to a set.
 
 	It is not an error to add an element that is already a member of the set.
diff --git a/libs/util/set.c b/libs/util/set.c
index baabe1057..9b59172a1 100644
--- a/libs/util/set.c
+++ b/libs/util/set.c
@@ -128,6 +128,30 @@ set_expand (set_t *set, unsigned size)
 		free (map);
 }
 
+void
+set_trim (set_t *set)
+{
+	if (set->map == set->defmap) {
+		return;
+	}
+	unsigned    words = SET_WORDS (set);
+
+	while (words > SET_DEFMAP_SIZE && !set->map[words - 1]) {
+		words--;
+		set->size -= SET_BITS;
+	}
+	if (words > SET_DEFMAP_SIZE) {
+		set_bits_t *map = realloc (set->map, words * sizeof (set_bits_t));
+		if (map && map != set->map) {
+			set->map = map;
+		}
+	} else {
+		memcpy (set->defmap, set->map, sizeof (set->defmap));
+		free (set->map);
+		set->map = set->defmap;
+	}
+}
+
 inline set_t *
 set_new_size_r (set_pool_t *set_pool, unsigned size)
 {
@@ -204,13 +228,14 @@ _set_union (set_t *dst, const set_t *src)
 static set_t *
 _set_intersection (set_t *dst, const set_t *src)
 {
-	unsigned    size;
+	unsigned    words;
 	unsigned    i;
 
-	size = max (dst->size, src->size);
-	set_expand (dst, size);
-	for (i = 0; i < SET_WORDS (src); i++)
+	words = min (SET_WORDS (dst), SET_WORDS (src));
+	for (i = 0; i < words; i++)
 		dst->map[i] &= src->map[i];
+	// if dst is larger than src, then none of the excess elements in dst
+	// can be in the intersection
 	for ( ; i < SET_WORDS (dst); i++)
 		dst->map[i] = 0;
 	return dst;
@@ -219,26 +244,37 @@ _set_intersection (set_t *dst, const set_t *src)
 static set_t *
 _set_difference (set_t *dst, const set_t *src)
 {
-	unsigned    size;
+	unsigned    words;
 	unsigned    i;
 
-	size = max (dst->size, src->size);
-	set_expand (dst, size);
-	for (i = 0; i < SET_WORDS (src); i++)
+	words = min (SET_WORDS (dst), SET_WORDS (src));
+	for (i = 0; i < words; i++)
 		dst->map[i] &= ~src->map[i];
+	// if src is larger than dst, excess elements in src cannot be in dst thus
+	// there is nothing to remove
+	// if dst is larger than src, there is nothing to remove regardless of what
+	// is in src
 	return dst;
 }
 
 static set_t *
 _set_reverse_difference (set_t *dst, const set_t *src)
 {
-	unsigned    size;
+	unsigned    words;
 	unsigned    i;
 
-	size = max (dst->size, src->size);
-	set_expand (dst, size);
-	for (i = 0; i < SET_WORDS (src); i++)
+	words = min (SET_WORDS (dst), SET_WORDS (src));
+	set_expand (dst, src->size);
+	for (i = 0; i < words; i++)
 		dst->map[i] = ~dst->map[i] & src->map[i];
+	// if src is larger than dst, then dst cannot remove the excess elements
+	// from src and thus the src elements must be copied
+	for ( ; i < SET_WORDS (src); i++)
+		dst->map[i] = src->map[i];
+	// if dst is larger than src, then the excess elements in dst must be
+	// removed
+	for ( ; i < SET_WORDS (dst); i++)
+		dst->map[i] = 0;
 	return dst;
 }
 
diff --git a/libs/util/test/test-set.c b/libs/util/test/test-set.c
index 4569c0046..412517d22 100644
--- a/libs/util/test/test-set.c
+++ b/libs/util/test/test-set.c
@@ -250,7 +250,30 @@ struct {
 	{make_not_1_2, make_0_1, set_union, check_count, 1, "{0 1 3 ...}"},
 	{make_not_1_2, make_0_1, set_intersection, check_count, 1, "{0}"},
 	{make_not_1_2, make_0_1, set_difference, check_count, 3, "{3 ...}"},
-	{make_not_1_2, make_0_1, set_reverse_difference, check_count, 1, "{1}"},
+	{make_not_1_2, make_0_1, set_reverse_difference, check_count, 1, "{1}"},//76
+	{make_SIZE, make_not_1_2, set_union, check_size, SIZE + SET_BITS,
+		"{0 3 ...}"},
+	{make_SIZE, make_not_1_2, set_intersection, check_size, SIZE + SET_BITS,
+		"{64}"},
+	{make_SIZE, make_not_1_2, set_difference, check_size, SIZE + SET_BITS,
+		"{}"},
+	{make_SIZE, make_not_1_2, set_reverse_difference, check_size,
+		SIZE + SET_BITS,
+		"{0 3 4 5 6 7 8 9 10 11 12 13 14 15"
+		" 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31"
+		" 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47"
+		" 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 65 ...}"
+	},//80
+	{make_not_1_2, make_SIZE, set_union, check_size, SIZE, "{0 3 ...}"},
+	{make_not_1_2, make_SIZE, set_intersection, check_size, SIZE + SET_BITS,
+		"{64}"},
+	{make_not_1_2, make_SIZE, set_difference, check_size, SIZE + SET_BITS,
+		"{0 3 4 5 6 7 8 9 10 11 12 13 14 15"
+		" 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31"
+		" 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47"
+		" 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 65 ...}"
+	},
+	{make_not_1_2, make_SIZE, set_reverse_difference, check_size, SIZE, "{}"},
 };
 #define num_tests (sizeof (tests) / sizeof (tests[0]))
 
@@ -270,6 +293,8 @@ main (int argc, const char **argv)
 	tests[9].str_expect = tests[5].str_expect;
 	tests[10].str_expect = tests[5].str_expect;
 	tests[11].str_expect = tests[5].str_expect;
+	tests[78].str_expect = tests[5].str_expect;
+	tests[82].str_expect = tests[5].str_expect;
 
 	str = dstring_new ();
 	for (i = 0; i < SIZE; i++) {
@@ -278,6 +303,15 @@ main (int argc, const char **argv)
 	dstring_appendstr (str, "}");
 	tests[6].str_expect = dstring_freeze (str);
 
+	str = dstring_new ();
+	dasprintf (str, "{0");
+	for (i = 3; i < SIZE; i++) {
+		dasprintf (str, " %zd", i);
+	}
+	dasprintf (str, " %zd ...}", SIZE + 1);
+	tests[80].str_expect = dstring_freeze (str);
+	tests[83].str_expect = tests[80].str_expect;
+
 	for (i = 0; i < num_tests; i++) {
 		set_t      *s1, *s2 = 0;
 		const char *set_str;