Could not increase JSON size

Hello everybody.

I have a problem with sending a JSON string via the ethernet.

Trying to set up a Mozilla Webthing, I ran into trouble when I had to increase the size of the JSON buffer.
The Mozilla Forum was a big help, but now I got stuck. Here is the forum discussion.

And here is the code I tried:

#define LARGE_JSON_DOCUMENT_SIZE 5555
#define SMALL_JSON_DOCUMENT_SIZE 2000

#define ARDUINOJSON_USE_LONG_LONG 1

#include <Arduino.h>
#include <stdio.h>
#include "Thing.h"
#include "EthernetWebThingAdapter.h"
//
//#include <ESP8266WiFi.h>
//#include "SoftwareSerial.h"

WebThingAdapter *adapter;


const char *dhtTypes[] = {"HumidityTempSensor", "Sensor", nullptr};
ThingDevice DHTtemp1("SensorDevice", "DHT temperature sensor plugged into a single pin", dhtTypes);
ThingProperty DHTprop("Temperature", "Digital Input Pin", NUMBER, "TemperatureProperty");

byte mac[] = { 0x90, 0xA2, 0xDA, 0x00, 0xFB, 0x80 };
IPAddress ip(192, 168, 0, 100);

void setup(void) {
  Serial.begin(9600);
  Serial.println("");
  Serial.println("");
  
  Ethernet.begin(mac, ip);
  Serial.print("Server started. IP: ");
  
  Serial.println(Ethernet.localIP());
  delay(100); // Give some time to initiate

  adapter = new WebThingAdapter("w25", Ethernet.localIP());
  DHTtemp1.addProperty(&DHTprop);
  adapter->addDevice(&DHTtemp1);
  adapter->begin();
  Serial.println("Starting HTTP server");
  Serial.print("http://");
  Serial.print(Ethernet.localIP());
  Serial.print("/things/");
  Serial.println(DHTtemp1.id);
}


void loop(void) {
  ThingPropertyValue levelValue;
  levelValue.number = random(0, 100);
  DHTprop.setValue(levelValue);
  adapter->update();
}

As soon as I increase the size of the JSON document
#define LARGE_JSON_DOCUMENT_SIZE 5555
#define SMALL_JSON_DOCUMENT_SIZE 2000
above a certain number, it won’t be visible any more.

Has anybody an idea where this could come from or seen something similar?

Many thanks and best regards.