File Size Difference with xlsm and xlsb

Hi, just a quick question for my knowledge. I have an Excel File that was 62Mb saved down as a xlsm file but i was looking to make this smaller and went through some forums that said that saving down as an xlsb would reduce file size by quite a bit.

I have tried this suggestion and my file size has reduced down to 35Mb, what i am looking to find out is what differences or changes could have been made to the file to get it down so low.

Any advice would be really appreciated.

Thanks