Difference between firmwares esp8266_4mb and esp8266 ?

It says 512/512. Is it the first or the second build ?

Yes, correct with setting from ESP8266_BOARD.py

Note: save() will compress the code and then write to flash

What about Minification?

Hmm, are you sure process.memory() is the correct output, because it say "total": 1700

What about your heap size?

>process.memory();
={ "free": 1562, "usage": 38, "total": 1600, "history": 20,
  "gc": 18, "gctime": 1.722 }

>require("ESP8266").getState();
={
  "sdkVersion": "2.0.0(656edbf)",
  "cpuFrequency": 160, "freeHeap": 10936, "maxCon": 10,
  "flashMap": "4MB:1024/1024",
  "flashKB": 4096,
  "flashChip": "0xe0 0x4016"
 }
> 

As more vars you configure as less heap memory you have :-)

So with GRAPHICS and 1600 var get "freeHeap": 10936 to work with.