Flutter: Image compression to specific size

Question

I'm working with an API which requires images lower or equals to 100 Kb. Server works only with png and jpeg extensions.

So I made a class that is responsible for compressing images. I use image for compression:

import 'dart:io';
import 'dart:math';

import 'package:flutter/material.dart';
import 'package:image/image.dart' as img;
import 'package:path/path.dart' as path;
import 'package:path_provider/path_provider.dart';

class ImageFileCompressor {
  static Future<File> compress(
    File image, {
    int targetSizeInBytes = 1024 * 1024,
  }) async {
    var fileToCompress = image;

    // The package works only with jpeg on iOS, 
    // so I have to check it first.
    final iosAndPng = _isIosAndPng(image);
    if (iosAndPng) {
      fileToCompress = await _convertPngToJpeg(image);
    }

    final compressed = await _compressImageToTargetSize(
      fileToCompress,
      targetSizeInBytes,
    );
    return compressed;
  }

  static int _calcSizeInBytes(File file) => file.lengthSync();


  static bool _isIosAndPng(File file) {
    final isIos = Platform.isIOS;
    final isPng = path.extension(file.path) == '.png';
    return isIos && isPng;
  }

  static Future<File> _convertPngToJpeg(File file) async {
    final image = await img.decodePngFile(file.path);
    final tempFile = await _tempFile(file, '.jpg');
    await img.encodeJpgFile(tempFile.path, image!);

    return tempFile;
  }

  static Future<File> _compressImageToTargetSize(
    File imageFile,
    int targetFileSize,
  ) async {
    var compressed = imageFile;
    var currentFileSize = _calcSizeInBytes(compressed);

   
    while (currentFileSize > targetFileSize) {
      compressed = await _compressImage(compressed);      

      // The problem appears here.
      // On particular point image stops compressing 
      // and returns size that equals or even greater 
      // than previous file size
      final newFileSize = _calcSizeInBytes(compressed);
      if (newFileSize >= currentFileSize) {
        throw Exception('Unable to compress image');
      }
      currentFileSize = newFileSize;
    }
    return compressed;
  }

  static Future<File> _compressImage(File imageFile) async {

    final image = await img.decodeJpgFile(imageFile.path);
    final tempFile = await _tempFile(imageFile, '.jpg');

    await img.encodeJpgFile(tempFile.path, image!, quality: 1);
    return File(tempFile.path);
  }

  static Future<File> _tempFile(File file, [String? extension]) async {
    final dir = await getTemporaryDirectory();
    final name = path.basenameWithoutExtension(file.path);
    final ext = extension ?? path.extension(file.path);
    final newName = '$name-${Random().nextInt(1000)}';
    final newPath = '${dir.path}/$newName$ext';
    debugPrint(newPath);
    return File(newPath);
  }
}

On particular point image stops compressing and returns size that equals or even greater than previous file size.

However some images that has lower initial size pass the loop successfully.

I also tried flutter_image_compress but the problem stays the same.

So at this point I am convinced that there is a limit of compression with this method and I should find some other ways to compress images.