diff --git a/elf/mold.h b/elf/mold.h
index 92de242920..3f826ac264 100644
--- a/elf/mold.h
+++ b/elf/mold.h
@@ -124,8 +124,10 @@ class Thunk<E> {
 };
 
 struct ThunkRef {
-  i16 thunk_idx = -1;
-  i16 sym_idx = -1;
+  static constexpr i64 MAX_SYM_IDX = (1 << 17) - 1;
+
+  i32 thunk_idx : 14 = -1;
+  i32 sym_idx : 18 = -1;
 };
 
 //
diff --git a/elf/thunks.cc b/elf/thunks.cc
index 79d1e52fbe..26b0d15cef 100644
--- a/elf/thunks.cc
+++ b/elf/thunks.cc
@@ -63,10 +63,10 @@ static consteval i64 max_distance() {
 // ARM64/ARM32/PPC, respectively.
 static constexpr i64 batch_size = max_distance() / 10;
 
-// We assume that a single thunk group is smaller than 100 KiB.
-static constexpr i64 max_thunk_size = 102400;
+// We assume that a single thunk group is smaller than 900 KiB.
+static constexpr i64 max_thunk_size = 900 * 1024;
 
-static_assert(max_thunk_size / E::thunk_size < INT16_MAX);
+static_assert(max_thunk_size / E::thunk_size < ThunkRef::MAX_SYM_IDX);
 
 template <typename E>
 static bool is_reachable(Context<E> &ctx, InputSection<E> &isec,
diff --git a/test/elf/range-extension-thunk3.sh b/test/elf/range-extension-thunk3.sh
new file mode 100755
index 0000000000..bb45e594fe
--- /dev/null
+++ b/test/elf/range-extension-thunk3.sh
@@ -0,0 +1,17 @@
+#!/bin/bash
+. $(dirname $0)/common.inc
+
+[ $MACHINE = alpha ] && skip
+[ $MACHINE = sh4 ] && skip
+
+seq 1 10000 | sed 's/.*/void func\0() {}/' > $t/a.c
+$CC -B. -o $t/b.so -shared $t/a.c
+
+seq 1 10000 | sed 's/.*/void func\0();/' > $t/c.c
+echo 'int main() {' >> $t/c.c
+seq 1 10000 | sed 's/.*/func\0();/' >> $t/c.c
+echo '}' >> $t/c.c
+
+$CC -c -o $t/d.o $t/c.c
+$CC -B. -o $t/exe $t/d.o $t/b.so
+$QEMU $t/exe