Because my server node returns this error Error: request entity too large?

I try to login with github but in the Node backend when I do the exchange of the code that returns me git for callback for an access_token I have this error

Error: request entity too large

fetch('https://github.com/login/oauth/access_token/', {
    method: 'POST',
    client_id: 'xxxx',
    client_secret: 'xxxx',
    code: req.params.code,
    accept: 'json',
}).then(function(res) {
    return res.json();
}).then(function(body) {
    res.json(body);
});

This returns if done by POSTMAN eg something like this

access_token=e5248cf7809a8f3da6f4c730bf80af12ef32c3cc&scope=user&token_type=bearer