Grayscale image activity file size question

Hello, I’m using the “Image to Grayscale” and I see that a converted color image originaly 160Kb size generate a 850Kb file. Another one orginaly 3430Kb generates a 10986Kb size.
Normally, converting in grayscale should not lead to such number. Does someone has any clue ?

Thanks !

notice that the SB.Image.Activity package by Santan Barnwal convert to gray scale activity generate (logically) a lighter file than the original in color.