problem with size of sof file

I'm using quartus II, in my project I have a sof file of a size of 2412 Ko,
I don't even modificate anything in my quartus direcotry project
and when I start assembler to generate a new sof file, I should
have the same the same size for my new sof file, but I have a
new sof file of 3459 Ko. So, I wanted to know why I have this problem,
and if there is a way or an option that could make me generate
a sof file with the wanted size ?