diff --git a/rugnux/RugnuxJointSweeps.cpp b/rugnux/RugnuxJointSweeps.cpp
index c6a6253e0..db0a59943 100644
--- a/rugnux/RugnuxJointSweeps.cpp
+++ b/rugnux/RugnuxJointSweeps.cpp
@@ -574,11 +574,24 @@ JointMergeInput BuildJointMergeInput(const std::vector<JointMemberSweep> &sweeps
         out.outcomes.resize(first + sweep.n_frames);
         for (int64_t f = 0; f < sweep.n_frames; ++f)
             out.outcomes[first + f].latt = lattice;
+        const auto usable = [](const ScaledFull &full) {
+            return std::isfinite(full.I) && std::isfinite(full.var_bkg) && full.var_per_I > 0.0f && full.d > 0.0f;
+        };
+        const auto frame_of = [&](const ScaledFull &full) {
+            return std::clamp<int64_t>(std::llround(full.image_number), 0, sweep.n_frames - 1);
+        };
+        // Each frame's reflections reserved at their count first: grown one by one, a large cell's
+        // sweeps held up to twice their size in spare capacity.
+        std::vector<size_t> count(sweep.n_frames, 0);
+        for (const ScaledFull &full : sweep.fulls)
+            if (usable(full))
+                count[frame_of(full)]++;
+        for (int64_t f = 0; f < sweep.n_frames; ++f)
+            out.outcomes[first + f].reflections.reserve(count[f]);
         for (const ScaledFull &full : sweep.fulls) {
-            if (!std::isfinite(full.I) || !std::isfinite(full.var_bkg) || !(full.var_per_I > 0.0f)
-                || !(full.d > 0.0f))
+            if (!usable(full))
                 continue;
-            const auto frame = std::clamp<int64_t>(std::llround(full.image_number), 0, sweep.n_frames - 1);
+            const auto frame = frame_of(full);
             const gemmi::Op::Miller hkl = rel.cob.apply_to_hkl({{full.h, full.k, full.l}});
             // A full as the merge takes a pre-combined observation, so that the combine rebuilds the variance
             // its own sweep's merge gave it: a count c of intensity I/c at correction c = var_per_I is a
diff --git a/rugnux/rugnux_cli.cpp b/rugnux/rugnux_cli.cpp
index 675567094..9ca68f76a 100644
--- a/rugnux/rugnux_cli.cpp
+++ b/rugnux/rugnux_cli.cpp
@@ -4262,9 +4262,10 @@ int RugnuxCli::RunJointSweeps(const RugnuxSweepPlan &plan) {
     JointMergeInput input = BuildJointMergeInput(members, relation);
     // The joint merge works on its own copy of the fulls (input.outcomes): the members' go, so that a large
     // cell's sweeps are not held twice.
+    // (Assigned an empty vector, not {}: that is an assignment from an empty list, which keeps the memory.)
     for (JointMemberSweep &m : members) {
-        m.fulls = {};
-        m.merged = {};
+        m.fulls = std::vector<ScaledFull>();
+        m.merged = std::vector<MergedReflection>();
     }
     if (input.segments.size() < 2)
         logger.Warning("Only {} sweep goes into the joint merge", input.segments.size());