Also the space taken up by the app includes anything cached, in the case of Messenger that's any messages/photos that came via Messenger stored on the device for offline viewing.
My phone is Android and the Messenger download is 49MB, on the phone currently takes up 173MB.
Facebook itself is even worse, when they updated it to store offline content about 12-18 months ago, it went from taking up 40-50MB to these days nearly 400.