DropDownMenuItem seems to be constrained to the width of the DropDownButton

main.dart

import 'package:flutter/material.dart';

final Color darkBlue = Color(0xFFFFFFF);

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      theme: ThemeData.dark().copyWith(scaffoldBackgroundColor: darkBlue),
      debugShowCheckedModeBanner: false,
      home: Scaffold(
        body: Center(
          child: WhereToFindWidget(),
        ),
      ),
    );
  }
}


class WhereToFindWidget extends StatefulWidget {
  const WhereToFindWidget({Key? key})
      : super(key: key);

  @override
  _WhereToFindWidgetState createState() => _WhereToFindWidgetState();
}

class _WhereToFindWidgetState extends State<WhereToFindWidget> {
  late DropDownCountry currentCountry;

  final List<DropDownCountry> _countrys = countryCodes.entries
      .map<DropDownCountry>((entry) => DropDownCountry(
          countryCode: entry.key,
          countryName: entry.value[0],
          countryEmoji: entry.value[1]))
      .toList()
        ..sort((a, b) => a.countryName.compareTo(b.countryName));

  @override
  void initState() {
    super.initState();
    currentCountry = _countrys.firstWhere(
        (element) => element.countryCode == 'US',
        );
  }

  @override
  Widget build(BuildContext context) {
  
   return Center(
      child: 
           DropdownButtonHideUnderline(
            child: DropdownButton<DropDownCountry>(
              value: currentCountry,
              isDense: true,
              dropdownColor: Color(0xFF222C3E),
              selectedItemBuilder: (BuildContext context) {
                return _countrys.map<Widget>((dropDownCountry) {
                  return Text(
                    dropDownCountry.countryEmoji,
                  );
                }).toList();
              },
              onChanged: (DropDownCountry? newValue) {
                setState(() {
                  currentCountry = newValue!;
                });
              },
              items: _countrys.map((DropDownCountry dropDownCountry) {
                return DropdownMenuItem<DropDownCountry>(
                  value: dropDownCountry,
                  child: Container(
                    decoration: BoxDecoration(color: Color(0xFF222C3E)),
                    child: Text(
                      dropDownCountry.countryEmoji +
                          ' ' +
                          dropDownCountry.countryName,
                      softWrap: false,
                    ),
                  ),
                );
              }).toList(),
            ),
          ),
    );
  }
}


class DropDownCountry  {
  const DropDownCountry({
    required this.countryCode,
    required this.countryName,
    required this.countryEmoji,
  });

  final String countryCode;
  final String countryName;
  final String countryEmoji;


}

const Map<String, List<String>> countryCodes = {
  'AR': ['Argentina', '🇦🇷'],
  'AT': ['Austria', '🇦🇹'],
  'AU': ['Australia', '🇦🇺'],
  'BE': ['Belgium', '🇧🇪'],
  'BR': ['Brazil', '🇧🇷'],
  'CA': ['Canada', '🇨🇦'],
  'CH': ['Switzerland', '🇨🇭'],
  'CL': ['Chile', '🇨🇱'],
  'CO': ['Colombia', '🇨🇴'],
  'CZ': ['Czechia', '🇨🇿'],
  'DE': ['Germany', '🇩🇪'],
  'DK': ['Denmark', '🇩🇰'],
  'EC': ['Ecuador', '🇪🇨'],
  'EE': ['Estonia', '🇪🇪'],
  'ES': ['Spain', '🇪🇸'],
  'FI': ['Finland', '🇫🇮'],
  'FR': ['France', '🇫🇷'],
  'GB': ['United Kingdom', '🇬🇧'],
  'GR': ['Greece', '🇬🇷'],
  'HU': ['Hungary', '🇭🇺'],
  'ID': ['Indonesia', '🇮🇩'],
  'IE': ['Ireland', '🇮🇪'],
  'IN': ['India', '🇮🇳'],
  'IT': ['Italy', '🇮🇹'],
  'JP': ['Japan', '🇯🇵'],
  'KR': ['Korea', '🇰🇷'],
  'LT': ['Lithuania', '🇱🇹'],
  'LV': ['Latvia', '🇱🇻'],
  'MX': ['Mexico', '🇲🇽'],
  'MY': ['Malaysia', '🇲🇾'],
  'NL': ['Netherlands', '🇳🇱'],
  'NO': ['Norway', '🇳🇴'],
  'NZ': ['New Zealand', '🇳🇿'],
  'PE': ['Peru', '🇵🇪'],
  'PH': ['Philippines', '🇵🇭'],
  'PL': ['Poland', '🇵🇱'],
  'PT': ['Portugal', '🇵🇹'],
  'RO': ['Romania', '🇷🇴'],
  'RU': ['Russia', '🇷🇺'],
  'SE': ['Sweden', '🇸🇪'],
  'SG': ['Singapore', '🇸🇬'],
  'TH': ['Thailand', '🇹🇭'],
  'TU': ['Turkey', '🇹🇷'],
  'US': ['United States', '🇺🇸'],
  'VE': ['Venezuela', '🇻🇪'],
  'ZA': ['South Africa', '🇿🇦']
};