diff --git a/quinn-proto/src/connection/mod.rs b/quinn-proto/src/connection/mod.rs
index 79a47ac7f..57940a46f 100644
--- a/quinn-proto/src/connection/mod.rs
+++ b/quinn-proto/src/connection/mod.rs
@@ -759,17 +759,18 @@ impl Connection {
                     "ACKs should leave space for ConnectionClose"
                 );
                 if buf.len() + frame::ConnectionClose::SIZE_BOUND < builder.max_size {
+                    let max_frame_size = builder.max_size - buf.len();
                     match self.state {
                         State::Closed(state::Closed { ref reason }) => {
                             if space_id == SpaceId::Data || reason.is_transport_layer() {
-                                reason.encode(buf, builder.max_size)
+                                reason.encode(buf, max_frame_size)
                             } else {
                                 frame::ConnectionClose {
                                     error_code: TransportErrorCode::APPLICATION_ERROR,
                                     frame_type: None,
                                     reason: Bytes::new(),
                                 }
-                                .encode(buf, builder.max_size)
+                                .encode(buf, max_frame_size)
                             }
                         }
                         State::Draining => frame::ConnectionClose {
@@ -777,7 +778,7 @@ impl Connection {
                             frame_type: None,
                             reason: Bytes::new(),
                         }
-                        .encode(buf, builder.max_size),
+                        .encode(buf, max_frame_size),
                         _ => unreachable!(
                             "tried to make a close packet when the connection wasn't closed"
                         ),
@@ -829,13 +830,8 @@ impl Connection {
                 }
             }
 
-            let sent = self.populate_packet(
-                now,
-                space_id,
-                buf,
-                buf_capacity - builder.tag_len,
-                builder.exact_number,
-            );
+            let sent =
+                self.populate_packet(now, space_id, buf, builder.max_size, builder.exact_number);
 
             // ACK-only packets should only be sent when explicitly allowed. If we write them due
             // to any other reason, there is a bug which leads to one component announcing write
diff --git a/quinn-proto/src/connection/packet_builder.rs b/quinn-proto/src/connection/packet_builder.rs
index c53d09a2d..30a62387f 100644
--- a/quinn-proto/src/connection/packet_builder.rs
+++ b/quinn-proto/src/connection/packet_builder.rs
@@ -18,7 +18,11 @@ pub(super) struct PacketBuilder {
     pub(super) ack_eliciting: bool,
     pub(super) exact_number: u64,
     pub(super) short_header: bool,
+    /// Smallest absolute position in the associated buffer that must be occupied by this packet's
+    /// frames
     pub(super) min_size: usize,
+    /// Largest absolute position in the associated buffer that may be occupied by this packet's
+    /// frames
     pub(super) max_size: usize,
     pub(super) tag_len: usize,
     pub(super) span: tracing::Span,
@@ -147,7 +151,7 @@ impl PacketBuilder {
             buffer.len() + (sample_size + 4).saturating_sub(number.len() + tag_len),
             partial_encode.start + conn.rem_cids.active().len() + 6,
         );
-        let max_size = buffer_capacity - partial_encode.start - partial_encode.header_len - tag_len;
+        let max_size = buffer_capacity - tag_len;
 
         Some(Self {
             datagram_start,