the data are not accessible with some tools by other apps but the system, this is by design and can be regarded as a security feature.
to cleanup space on the device, you can delete unused chats and/or messages. in fact, we’re currently working to make this a bit smoother, eg. offer a method to “clean” a chat without deleting it and also having some options to delete old messags automatically from the device.
but also, don’t overestimate the amout of space that can be freed in your example screenshot: even if all data are deleted, app size will probably still be about 83mb (not sure what android is doing here exactly, the apk is smaller)