I have files in .txt with the total weight of 457 mb. I need to save it in 170 mb of space of the kindle oasis 2019.
I haven't tested this, but my first guess is just dumping it into a DOCX file and then using Calibre or Kindle Previewer to make a mobi file. That compresses the files significantly. Docx is a very efficient binary file -- much more efficient than txt or even html.
Incidentally when you create a mobi file, the file size of the created file is twice what Kindle actually uses. So a 200MB Mobi file will probably only appear to the Kindle app to be 100 MB.
Practically speaking, larger files take a while to load, so there's value in splitting them up into separate files. But I think if the source docx file is 100 MB, then the created mobi file will be 200 mb, but the Kindle app will only download 100mb from the cloud.
I am assuming that you are uploading the file to Kindle Cloud via email and then downloading the ebook onto your Kindle device? If you are sideloading, then the file size will still be 2x of what it needs to be.
To summarize: use DOCX for the source and upload to your Kindle account and then have Kindle cloud send it to your device.