Expirment to generate JSON and Proto data and then gzip them to compare the results.

main.go

package main

import (
    "bufio"
    "bytes"
    "compress/gzip"
    "encoding/json"
    "fmt"
    "math/rand/v2"
    "os"
    "text/tabwriter"

    "google.golang.org/protobuf/proto"
)

const (
    usernamesFile = "usernames.txt"
)

var UserCounts = [...]int{1, 10, 100, 1000, 10000, 100000, 1000000}

func main() {
    names := readUsername(usernamesFile)

    usersList := UsersProto{
        Users: []*UserProto{},
    }

    writer := tabwriter.NewWriter(os.Stdout, 0, 0, 1, ' ', tabwriter.Debug)
    fmt.Fprintln(writer, "Users\tJSON Size\tGzipped JSON Size\tJSON Gzip size % \tProto Size\tGzipped Proto Size\tProto Gzip size %\tGzip Diff (JSON - Proto)")
    for _, num := range UserCounts {
        for i := 0; i < num; i++ {

            name := names[rand.IntN(len(names))]
            user := UserProto{
                Name:  name,
                Age:   rand.Int32N(91) + 10,
                Email: fmt.Sprintf("%s@gmail.com", name),
            }

            usersList.Users = append(usersList.Users, &user)
        }

        jsonData, _ := json.Marshal(usersList)
        protData, _ := proto.Marshal(&usersList)
        gzippedProtoSize := gzipDataAndReturnSize(protData)
        gzippedJsonSize := gzipDataAndReturnSize(jsonData)

        jsonReduction := float64(gzippedJsonSize) / float64(len(jsonData)) * 100
        protoReduction := float64(gzippedProtoSize) / float64(len(protData)) * 100
        diff := gzippedJsonSize - gzippedProtoSize

        fmt.Fprintf(writer, "%d\t%s\t%s\t%.0f%%\t%s\t%s\t%.0f%%\t%s\n",
            num,
            humanReadableSize(len(jsonData)),
            humanReadableSize(gzippedJsonSize),
            jsonReduction,
            humanReadableSize(len(protData)),
            humanReadableSize(gzippedProtoSize),
            protoReduction,
            humanReadableSize(diff),
        )
    }
    writer.Flush()
}

// humanReadableSize returns a human-readable size string.
// e.g. 1024 -> 1 KB
// e.g. 1048576 -> 1 MB
func humanReadableSize(bytes int) string {
    const unit = 1024
    absBytes := abs(bytes)
    if absBytes < unit {
        return fmt.Sprintf("%dB", absBytes)
    }
    div, exp := int64(unit), 0
    for n := absBytes / unit; n >= unit; n /= unit {
        div *= unit
        exp++
    }
    return fmt.Sprintf("%.1f%cB", float64(signOf(bytes, absBytes))/float64(div), "KMGTPE"[exp])
}

// abs returns the absolute value of x.
func abs(x int) int {
    if x < 0 {
        return -x
    }
    return x
}

// signOf returns the sign of the original number applied to the applySignTo number.
func signOf(original int, applySignTo int) int {
    if (original < 0) == (applySignTo < 0) {
        return applySignTo
    }
    return -applySignTo
}

func readUsername(fileName string) []string {
    file, _ := os.Open(fileName)
    defer file.Close()

    scanner := bufio.NewScanner(file)
    var names []string
    for scanner.Scan() {
        names = append(names, scanner.Text())
    }
    return names
}

func gzipDataAndReturnSize(data []byte) int {
    var buf bytes.Buffer
    gw := gzip.NewWriter(&buf)
    gw.Write(data)
    gw.Close()

    return buf.Len()
}

result.txt

Users   |JSON Size |Gzipped JSON Size |JSON Gzip size %  |Proto Size |Gzipped Proto Size |Proto Gzip size % |Gzip Diff (JSON - Proto)
1       |75B       |85B               |113%              |40B        |55B                |138%              |30B
10      |673B      |232B              |34%               |398B       |216B               |54%               |16B
100     |6.3KB     |1.4KB             |22%               |3.7KB      |1.4KB              |37%               |12B
1000    |62.5KB    |11.8KB            |19%               |36.5KB     |12.3KB             |34%               |542B
10000   |624.9KB   |115.7KB           |19%               |364.3KB    |121.7KB            |33%               |-6.0KB
100000  |6.1MB     |1.1MB             |18%               |3.6MB      |1.2MB              |33%               |-60.7KB
1000000 |61.1MB    |11.3MB            |18%               |35.7MB     |11.9MB             |33%               |-603.8KB