diff --git a/builder/build.go b/builder/build.go
index 64a1fde61d..57b67ed455 100644
--- a/builder/build.go
+++ b/builder/build.go
@@ -61,6 +61,10 @@ type BuildResult struct {
 	// correctly printing test results: the import path isn't always the same as
 	// the path listed on the command line.
 	ImportPath string
+
+	// Map from path to package name. It is needed to attribute binary size to
+	// the right Go package.
+	PackagePathMap map[string]string
 }
 
 // packageAction is the struct that is serialized to JSON and hashed, to work as
@@ -242,6 +246,12 @@ func Build(pkgName, outpath, tmpdir string, config *compileopts.Config) (BuildRe
 		return result, err
 	}
 
+	// Store which filesystem paths map to which package name.
+	result.PackagePathMap = make(map[string]string, len(lprogram.Packages))
+	for _, pkg := range lprogram.Sorted() {
+		result.PackagePathMap[pkg.OriginalDir()] = pkg.Pkg.Path()
+	}
+
 	// Create the *ssa.Program. This does not yet build the entire SSA of the
 	// program so it's pretty fast and doesn't need to be parallelized.
 	program := lprogram.LoadSSA()
@@ -916,11 +926,7 @@ func Build(pkgName, outpath, tmpdir string, config *compileopts.Config) (BuildRe
 
 			// Print code size if requested.
 			if config.Options.PrintSizes == "short" || config.Options.PrintSizes == "full" {
-				packagePathMap := make(map[string]string, len(lprogram.Packages))
-				for _, pkg := range lprogram.Sorted() {
-					packagePathMap[pkg.OriginalDir()] = pkg.Pkg.Path()
-				}
-				sizes, err := loadProgramSize(result.Executable, packagePathMap)
+				sizes, err := loadProgramSize(result.Executable, result.PackagePathMap)
 				if err != nil {
 					return err
 				}
diff --git a/builder/sizes_test.go b/builder/sizes_test.go
index bda5e07602..a1be28f274 100644
--- a/builder/sizes_test.go
+++ b/builder/sizes_test.go
@@ -1,6 +1,7 @@
 package builder
 
 import (
+	"regexp"
 	"runtime"
 	"testing"
 	"time"
@@ -55,26 +56,7 @@ func TestBinarySize(t *testing.T) {
 			t.Parallel()
 
 			// Build the binary.
-			options := compileopts.Options{
-				Target:        tc.target,
-				Opt:           "z",
-				Semaphore:     sema,
-				InterpTimeout: 60 * time.Second,
-				Debug:         true,
-				VerifyIR:      true,
-			}
-			target, err := compileopts.LoadTarget(&options)
-			if err != nil {
-				t.Fatal("could not load target:", err)
-			}
-			config := &compileopts.Config{
-				Options: &options,
-				Target:  target,
-			}
-			result, err := Build(tc.path, "", t.TempDir(), config)
-			if err != nil {
-				t.Fatal("could not build:", err)
-			}
+			result := buildBinary(t, tc.target, tc.path)
 
 			// Check whether the size of the binary matches the expected size.
 			sizes, err := loadProgramSize(result.Executable, nil)
@@ -90,3 +72,69 @@ func TestBinarySize(t *testing.T) {
 		})
 	}
 }
+
+// Check that the -size=full flag attributes binary size to the correct package
+// without filesystem paths and things like that.
+func TestSizeFull(t *testing.T) {
+	tests := []string{
+		"microbit",
+		"wasip1",
+	}
+
+	libMatch := regexp.MustCompile(`^C [a-z -]+$`) // example: "C interrupt vector"
+	pkgMatch := regexp.MustCompile(`^[a-z/]+$`)    // example: "internal/task"
+
+	for _, target := range tests {
+		target := target
+		t.Run(target, func(t *testing.T) {
+			t.Parallel()
+
+			// Build the binary.
+			result := buildBinary(t, target, "examples/serial")
+
+			// Check whether the binary doesn't contain any unexpected package
+			// names.
+			sizes, err := loadProgramSize(result.Executable, result.PackagePathMap)
+			if err != nil {
+				t.Fatal("could not read program size:", err)
+			}
+			for _, pkg := range sizes.sortedPackageNames() {
+				if pkg == "(padding)" || pkg == "(unknown)" {
+					// TODO: correctly attribute all unknown binary size.
+					continue
+				}
+				if libMatch.MatchString(pkg) {
+					continue
+				}
+				if pkgMatch.MatchString(pkg) {
+					continue
+				}
+				t.Error("unexpected package name in size output:", pkg)
+			}
+		})
+	}
+}
+
+func buildBinary(t *testing.T, targetString, pkgName string) BuildResult {
+	options := compileopts.Options{
+		Target:        targetString,
+		Opt:           "z",
+		Semaphore:     sema,
+		InterpTimeout: 60 * time.Second,
+		Debug:         true,
+		VerifyIR:      true,
+	}
+	target, err := compileopts.LoadTarget(&options)
+	if err != nil {
+		t.Fatal("could not load target:", err)
+	}
+	config := &compileopts.Config{
+		Options: &options,
+		Target:  target,
+	}
+	result, err := Build(pkgName, "", t.TempDir(), config)
+	if err != nil {
+		t.Fatal("could not build:", err)
+	}
+	return result
+}