diff --git a/butane/config/openshift/v4_18/translate_test.go b/butane/config/openshift/v4_18/translate_test.go
index 8295ea2d2..5b7dbe254 100644
--- a/butane/config/openshift/v4_18/translate_test.go
+++ b/butane/config/openshift/v4_18/translate_test.go
@@ -513,3 +513,39 @@ func TestValidateSupport(t *testing.T) {
 		})
 	}
 }
+
+// TestMachineConfigYAMLLargeSizeMiB is the OCPBUGS-114878 / #2309 regression:
+// MachineConfig YAML must encode sizeMiB >= 1e6 as a plain integer.
+func TestMachineConfigYAMLLargeSizeMiB(t *testing.T) {
+	in := `variant: openshift
+version: 4.18.0
+metadata:
+  name: 98-master-partition
+  labels:
+    machineconfiguration.openshift.io/role: master
+storage:
+  disks:
+    - device: /dev/disk/by-id/virtio-targetdisk
+      partitions:
+        - number: 4
+          label: root
+          size_mib: 8389000
+          resize: true
+        - number: 5
+          label: var
+          size_mib: 0
+        - number: 6
+          label: odf-1
+          size_mib: 1231872
+`
+	out, r, err := ToConfigBytes([]byte(in), common.TranslateBytesOptions{})
+	assert.NoError(t, err)
+	assert.False(t, r.IsFatal())
+	got := string(out)
+	assert.Contains(t, got, "sizeMiB: 8389000")
+	assert.Contains(t, got, "sizeMiB: 1231872")
+	assert.Contains(t, got, "sizeMiB: 0")
+	assert.NotContains(t, got, "8.389e+06")
+	assert.NotContains(t, got, "1.231872e+06")
+	assert.NotContains(t, got, "e+")
+}
diff --git a/butane/config/util/util.go b/butane/config/util/util.go
index fc6de99db..019dab082 100644
--- a/butane/config/util/util.go
+++ b/butane/config/util/util.go
@@ -155,8 +155,8 @@ func TranslateBytesYAML(input []byte, container interface{}, translateMethod str
 		return jsonCfg, r, err
 	}
 
-	var ifaceCfg interface{}
-	if err := json.Unmarshal(jsonCfg, &ifaceCfg); err != nil {
+	ifaceCfg, err := unmarshalJSONForYAML(jsonCfg)
+	if err != nil {
 		return []byte{}, r, err
 	}
 
@@ -174,6 +174,49 @@ func TranslateBytesYAML(input []byte, container interface{}, translateMethod str
 	return yamlCfg, r, err
 }
 
+// unmarshalJSONForYAML decodes JSON into a generic structure suitable for
+// YAML encoding. Integers are preserved as int64 so yaml.v3 does not emit
+// scientific notation for values >= 1e6 (e.g. sizeMiB: 8.389e+06).
+func unmarshalJSONForYAML(jsonCfg []byte) (interface{}, error) {
+	dec := json.NewDecoder(bytes.NewReader(jsonCfg))
+	dec.UseNumber()
+	var ifaceCfg interface{}
+	if err := dec.Decode(&ifaceCfg); err != nil {
+		return nil, err
+	}
+	return convertJSONNumbers(ifaceCfg), nil
+}
+
+// convertJSONNumbers walks a decoded JSON tree and replaces json.Number
+// values with int64 when possible, otherwise float64. json.Unmarshal into
+// interface{} otherwise uses float64, and yaml.v3 then formats values >= 1e6
+// with strconv.FormatFloat 'g' as scientific notation.
+func convertJSONNumbers(v interface{}) interface{} {
+	switch x := v.(type) {
+	case map[string]interface{}:
+		for k, val := range x {
+			x[k] = convertJSONNumbers(val)
+		}
+		return x
+	case []interface{}:
+		for i, val := range x {
+			x[i] = convertJSONNumbers(val)
+		}
+		return x
+	case json.Number:
+		if i, err := x.Int64(); err == nil {
+			return i
+		}
+		f, err := x.Float64()
+		if err != nil {
+			return x.String()
+		}
+		return f
+	default:
+		return v
+	}
+}
+
 // Report an ErrFieldElided warning for any non-zero top-level fields in the
 // specified output struct.  The caller will probably want to use
 // translate.PrefixReport() to reparent the report into the right place in
diff --git a/butane/config/util/util_test.go b/butane/config/util/util_test.go
index fc744d92c..9d138e540 100644
--- a/butane/config/util/util_test.go
+++ b/butane/config/util/util_test.go
@@ -15,7 +15,9 @@
 package util
 
 import (
+	"bytes"
 	"fmt"
+	"strings"
 	"testing"
 
 	"github.com/coreos/ignition/v2/butane/config/common"
@@ -24,6 +26,7 @@ import (
 	"github.com/coreos/vcontext/path"
 	"github.com/coreos/vcontext/report"
 	"github.com/stretchr/testify/assert"
+	"gopkg.in/yaml.v3"
 )
 
 func TestSnake(t *testing.T) {
@@ -119,3 +122,22 @@ func TestTranslateReportPaths(t *testing.T) {
 	assert.Equal(t, makeReport(false), r, "TranslateReportPaths changed original report")
 	assert.Equal(t, makeReport(true), r2, "TranslateReportPaths returned incorrect report")
 }
+
+func TestUnmarshalJSONForYAMLIntegers(t *testing.T) {
+	jsonCfg := []byte(`{"sizeMiB":8389000,"startMiB":2048,"values":[1000000],"ratio":1.5}`)
+	v, err := unmarshalJSONForYAML(jsonCfg)
+	assert.NoError(t, err)
+
+	var buf bytes.Buffer
+	enc := yaml.NewEncoder(&buf)
+	enc.SetIndent(2)
+	assert.NoError(t, enc.Encode(v))
+	assert.NoError(t, enc.Close())
+	out := buf.String()
+
+	assert.Contains(t, out, "sizeMiB: 8389000")
+	assert.Contains(t, out, "startMiB: 2048")
+	assert.Contains(t, out, "- 1000000")
+	assert.Contains(t, out, "ratio: 1.5")
+	assert.False(t, strings.Contains(out, "e+"), "YAML must not use scientific notation: %s", out)
+}
diff --git a/docs/release-notes.md b/docs/release-notes.md
index e4015a63f..ed02b743f 100644
--- a/docs/release-notes.md
+++ b/docs/release-notes.md
@@ -14,6 +14,10 @@ nav_order: 9
 
 ### Bug fixes
 
+- Encode large integer fields such as `sizeMiB` as decimal numbers in
+  MachineConfig YAML, instead of scientific notation
+  ([#2309](https://github.com/coreos/ignition/issues/2309))
+  ([OCPBUGS-114878](https://redhat.atlassian.net/browse/OCPBUGS-114878))
 
 ## Ignition 2.27.0 (2026-08-26)