The bin file size variations

I've noticed that under the same ECU, some source files are 2MB while others are 4 or even 6MB. Are they all compatible with the same ECU? Can they be directly written into the ECU after resizing, or are they unable to match each other?

It mostly depend on how the file was read form the ECU. There a basically 3 ways read an ecu (OBD2, Bench, and BDM). For example in an obd read only the sectors you need to tune the car. While a BMD or bench read will get everything including IMMO data( usefully when you need to clone the ecu) This a rough exp but that is how i understand it. Playing with winols will expose you to how an ecu works internally and where all the chip set due. I recommend youtubeing ecu repairs and see how other read different times of chips form an ecu